Looking at the trajectory of Upward's literary life, they are apt choices. As a young writer at Cambridge in the 1920s he acquired a reputation based on a series of stories, written with Christopher Isherwood, featuring macabre scenes of provincial horror in the fictional English village of Mortmere. But just as important, his subsequent career as a writer was shaped, some have said fatally distorted, by his commitment to leftwing politics.

It was Stephen Spender who recalled that as a young man Auden was, for him, the "highest peak". For Auden himself it was Isherwood, and for Isherwood "there was a still further peak" - Upward. Katherine Bucknell, editor of Auden's juvenilia and Isherwood's diaries, says she first encountered Upward as "this rather mysterious higher authority behind Auden and Isherwood in the early 1930s. There are clear echoes of Mortmere in very early Auden where he picked up on that excitement and the macabre world that Upward and Isherwood had invented. And for Isherwood, Upward was the gold standard. He went on valuing Upward's opinion and sent every book to him. Isherwood never doubted that Upward's intellectual and creative abilities were authentic."

While Auden and Isherwood went on to literary superstardom, Upward, partly through choice and partly through circumstance, faded almost entirely from view. "We were called 'the Auden Generation'," he says, referring to the group of left-leaning writers who seemed to have defined what Auden called that, "low dishonest decade", the 1930s, "but I was generally left out. Or if they did include me it was to say I'd written a few unreadable books." In any case, says Upward, they weren't a movement like the surrealists in France, "although people have tried to make it a movement in retrospect".

Upward's first novel, Journey to the Border, was published in 1938, but he did not publish again for nearly 25 years, when In the Thirties, the first volume of his Spiral Ascent trilogy, came out. For much of this period Upward was better known for a controversial 1937 essay called "Sketch for a Marxist Interpretation of Literature" rather than for his fiction. "Everyone quoted me as saying that you can only write a good book if you have Marxist ideas," he says. "I hadn't actually quite said that. I also said you needed some skill. But it sort of hung around me."

While many of his fellow writers abandoned their youthful Marxism, Upward remained - and remains - sympathetic to communism. But his attempts to marry his art and his politics took its toll on both. For some time it looked unlikely that even the second volume of his trilogy would be published and the third eventually had to be underwritten by the Arts Council, and it only agreed after being publicly pressured by Isherwood and Spender.

When The Rotten Elements (1969) and No Home but the Struggle (1977) did emerge, they were at first ignored and then disparaged. Upward's attempt to find an appropriate style for his semi-autobiographical story about a poet's struggle with Marxism and art provoked the critic Samuel Hynes to call him "an arid, unimaginative and unreadable realist". Valentine Cunningham, in his comprehensive study, British Writers of the Thirties, declared that Upward would "scarcely be noticeable without his chums' repeated advertisements for his merits and importance". As for The Spiral Ascent, Cunningham said that "Upward dulled himself, as it were, into silence at the end of the 30s, and when he returned to fiction as a Communist party renegade with The Upward Spiral [sic] he had only this dull medium at his command, and his trilogy simply bores you."

Upward, perhaps understandably as Cunningham got the title of his trilogy wrong, says he regarded this sort of criticism with "contempt". But it is hard not to reflect that if Upward had died at, say, the same age as Auden, 66, these sort of judgments might have been definitive. But in the years since, and particularly over the past decade or so when the Enitharmon Press has reissued some of his out-of-print writing as well as his new stories, he has seen his work, and his place in 20th-century literature, reassessed.

A new collection of stories, A Renegade in Springtime, containing work written between 1929 and 2000, has recently been published. Peter Parker, Isherwood's biographer, says, "looking at this volume you do see that it is all of a piece. It isn't someone who started with these wacky stories and then gave it up to write a social realist political trilogy and then came back to it. You can see similar themes running right through the work." John Sutherland, Spender's biographer, says, "Trollope said 'it is dogged that does it', but while the life and career have been extraordinarily long, there is a lot more to Upward than just being the last man standing."

Edward Falaise Upward was born in Romford, Essex, in 1903. He had two brothers, both now dead, "one of whom unfortunately spent his whole life in a mental hospital," says Upward. "He had dementia praecox [schizophrenia], although he wasn't as dangerous as some can be." He also has one surviving younger sister who lives in a nursing home.

Their father, who was at university with Auden's father, was a doctor. "They were grocery importers," explains Upward, "but he was pushed into medicine by the family who wanted to distance themselves from 'trade'." There was, however, some tradition of writing. Allen Upward, a cousin, had written several bestselling books before committing suicide in 1926 by shooting himself with a rifle. Upward says his father would also rather have been a writer than a doctor, "and was very keen that I should do it. He read and criticised all my work and was very helpful. When my first novel got a bad review in some newspaper my father at once cancelled his order.

"After the war I had the greatest difficulty getting my second book published and when at last it was going to happen I told my father but he said 'I'm afraid it's too late.' He was ill and as a doctor he knew when he was going to die. But he said 'I think that calls for a cigar and a double whisky.' And I'm actually rather glad he didn't see it as there were things in it that I would rather he hadn't read."

Upward says that if his father had had his way he would have gone to a local school. "And my life would have been totally different. But my mother was very socially conscious. I think my father hated her. I remember when I refused to be confirmed she said that the religion didn't matter, but I'd get a better job if I was confirmed. So she insisted I was sent to Repton, and that's where I met Isherwood." Although Upward remembers joining in making a bonfire to celebrate the first world war armistice, he was already something of a rebel and objected to military training. "And school was the only time I ever felt really hungry," he recalls. "I once remember being given a jar of apricot jam which I ate in one go." Even now, while the Old Reptonian website commemorates Isherwood, there is no mention of Upward.

In 1922 he went up to Corpus Christi, Cambridge, to read history. His primary literary form was poetry and he went on to win the prestigious Chancellor's Medal. Isherwood joined him the following year and they immediately began what he describes as their "romancing" about their stories set around their fiction of Mortmere. "I would leave a story on his desk and he'd pick it up in the morning and vice versa," he explains. Isherwood later described the young Upward as "a pale, small, silent boy, strikingly handsome, with dark hair and blue eyes".

Although the heterosexual Upward and homosexual Isherwood never had an affair - both have commented that the absence of sexual attraction and rivalry helped prolong their friendship - Katherine Bucknell says there was an "erotic charge" between them. "But it was a love affair about the writing. Isherwood said he even got excited when he looked at Upward's handwriting. It was so thrilling to be swapping these crazy ideas. They wouldn't even bother to finish the stories. They would just get to the point in the story where the other person would be surprised or disgusted or nauseated and that would be enough."

Some of the stories featured pornographers and had titles such as "The Leviathan of the Urinals" and "The Loathly Succubus". Upward later destroyed most of this work, although some remnants were published as The Mortmere Stories in 1994 and this sort of humour still appeals to him. Peter Parker says, "just because Upward became a rather sternly doctrinaire Marxist, people assumed he was a rather solemn and austere figure. But in person he was always very entertaining and his letters to Isherwood are full of jokes and quips."

Upward and Isherwood's rebellious tendencies at university were directed against what they called the "poshocracy", a loose term that led them to mistrust almost anyone in authority, including politicians. When Upward later became known as a Marxist he was sometimes accused of being a member of the Cambridge spy ring - in fact he is seven or eight years older than them - but as an undergraduate, and in the years immediately after leaving Cambridge in 1925, he was not actively involved in politics. During the 1926 general strike he remembers being "against the government, of course", but when a girlfriend described the strike as "splendid" he recalls feeling irritated with her.

By this time Upward had begun his career as a teacher, at prep schools in Scotland and then Cornwall. He had continued to write and completed his most famous story, The Railway Accident, in 1929 although it wasn't published until 1949, with a preface from Isherwood and under the name of Allen Chalmers, the fictional name Isherwood gave Upward in his early autobiographical book, Lions and Shadows.

"Upward had an enormous reputation before he appeared in print," explains Peter Parker. "He'd decided that the Mortmere stuff wouldn't do and had destroyed it. The Railway Accident was published only because Isherwood pressured him, it was in America, under a pseudonym and with a preface saying he disowned it. And it was slightly bowdlerised - with the rape of a boy by a troop of boy scouts at a garden fete removed."

Even in the expurgated version the prose is robust with Upward describing gasometers as "like stumps of semi-amputated breasts" and noting during the train crash how the "coaches mounted like viciously copulating bulls". "People have called it surrealist," he says, "but it is more of a parody of surrealism." Auden, whom Upward had by this time met through Isherwood, was so taken with the story "that he actually read it to audiences," laughs Upward. "It was exceedingly indecent and I can't believe he got away with it." Upward later wrote that Auden had said the rape scene couldn't have been written by a homosexual. "I wished I had asked him why."

In 1932 Upward took a job teaching English at Alleyn's boys school in Dulwich, south London, where he stayed until his retirement to the Isle of Wight in 1961. He became the first head of the school's English department - until then it was not seen as an entirely proper subject - and says while he always taught at secondary school level, he would "have loved to have taught younger children whose imaginations I could have reached. Sometimes the older children would see the light, realising that Shakespeare was good, and I felt I had really scored something then."

He says at university he and Isherwood "never believed Shakespeare was any good. We thought it was one of those frauds that was imposed upon us. But then I read AC Bradley on Shakespeare and it converted me. Isherwood was disgusted with me for having sold out. Then he read Bradley and was converted as well." Upward is currently rereading Shakespeare's sonnets. "They do still give me the greatest pleasure".

Arthur Chandler, now Alleyn's school historian, was taught by Upward in the late 40s. "I remember him as having a very laid-back style, but he was firm and you listened. It was a bit peculiar to have some one extremely leftwing teaching at an English public school. I was told that one teacher was advised to put an Upward book in a plain wrapper if he wanted to read it in the common room." Upward concurs as to the school's tacit "don't tell-don't ask" line in regard to his politics. "They must have known," he says. "But in those days even using the word 'capitalism' was enough to damn you, so not much was said. But I carried on my political work in the evenings selling the Daily Worker, and politics was my salvation. I was miserable and couldn't have borne the life of a teacher without it."

He had visited the Soviet Union in the early 30s - "although we didn't really know what was happening there" - and began to attend cell meetings of the Communist party of Great Britain. "After a few years someone said it was about time I joined. I didn't feel worthy of it, but I did join." Upward didn't go to fight in the Spanish Civil War and says he felt guilty, "although the party didn't ask me to go. If it had I would have gone."

It was in the CPGB that he met Hilda Percival, a primary school teacher. They married in 1936 and were together until Hilda's death in 1995. They had two children: Christopher, who lectured in German at Aston university and translated correspondence between Marx and Engels, and Kathy, also a language teacher. Christopher suffered from multiple sclerosis. Upward recalls their last meeting on the Isle of Wight a few years ago, when his son told him it was unlikely they would see each other again. "He was writing a history of spelling and I said, 'for heaven's sake finish that last chapter. Don't be diverted'. But he got keen on something else and didn't." Christopher died last year and Upward's new book is dedicated to his memory.

Upward's debut novel, Journey to the Border (1938), in which a tutor breaks out of his despair by concluding that socialism is the only answer to the world's ills, was published by the Woolfs' Hogarth Press. Upward remembers Virginia Woolf smoking a cheroot. "I was surprised when they took the book, as Virginia had written attacking our lot [the left]. I often wondered whether the madness of my character in the book appealed to her. She had fits of madness where she heard the birds talking Greek and soon after that committed suicide."

When the second world war began Upward was 36. He didn't join the armed forces but later volunteered for the fire service. After the war he left the CPGB: he felt that the party, by backing the Labour government's drive for increased production, had ceased to be revolutionary. Although he never joined another political party he remained politically active, particularly in CND. "The party deteriorated in the same way as it did in the Soviet Union," he says. "In the end it became utterly un-communist. I often wondered how it would have worked if Lenin had lived on. But I still say to myself now, if I knew what Stalin was like and what Hitler was like, I would still have been on Stalin's side. He broke the Nazi army. Bad as Stalin was, he wasn't as bad as those who actually gassed the Jews."

This political crisis was intertwined with an ongoing creative crisis in matching his art to his world view. He says he wrote nothing worthwhile from 1938 to 1954. "I wanted to write but couldn't. And whatever I did write seemed like a foreign language. I had to ask people if it made any sense." In the early 50s he even took a year off work to solve the problem but failed.

Katherine Bucknell has seen drafts of his work from this period. "He would write the same paragraph over and over again. Changing a word and then crossing it out. It is painful to see how stressful it was for him. He had the conviction to lead his personal life according to principles that people talked up in the 30s and was determined to forego all of the privileges on offer to his social class and his education. His closest friends had gone on to become so famous, and he was a schoolteacher. Imagine how hard it was to have taken the year off to give himself a real chance, and still not be able to write."

For long periods of his time in the literary wilderness Upward's consistent left-wing stance left him estranged from his right-moving friends. He says that when Isherwood and Auden left for America in 1939 "I felt they seemed to say 'goody, goody, we needn't be anti-fascist and all that any more'. I wrote to them saying how disgusted I was."

His relationship with Auden was further soured by the latter's subsequent comments giving support for American involvement in Vietnam. "When he left England I said he had said goodbye to being a great poet and a genius," says Upward. "And I still believe that his best work was done when he was in England. In his later work he seemed afraid of any kind of emotion and was not as good as he could have been." Auden later withdrew, and then reinstated, a dedication to Upward of his poem "Helensburgh Ode", but the two men were not reconciled before Auden's death in 1973.

Bucknell says they essentially had different views on what art was for. "Auden felt he had to get away from the political role he had accidentally established for himself. There was a form of ventriloquism in Auden, he wrote a poem from the point of view of a communist, although he eventually did evolve a very serious and coherent philosophy of life that involved Christianity. And while Upward regretted the loss of the friendship, he stood his ground on principle. "

Upward's 1987 story, "At the Ferry Inn", is an unsettling account of a failed effort to be reconciled with an Auden-like figure. "It is about everyone meeting with their past self," says Margaret Drabble, who first encountered Upward's work when editing the Oxford Companion to English Literature in the early 80s. "And to his credit he doesn't allow himself the indulgence, as many lesser writers would, of justifying himself. Both [characters] behave rather oddly and are not reconciled. The story has a psychological truth to it, although it never took place. Upward is a part of literary history, he contributed to it and continues to contribute."

Upward says that his creative block eventually led to a sort of breakdown. He went to a Jungian therapist "but she saw that wasn't what I wanted and all I got was sleeping pills. But as a consequence of that breakdown I somehow became able to get rid of the CP [Communist party] and begin to write again." He had by now long concluded that his "fantastic stories" were inappropriate. "I wanted to write something more serious. I thought that an intelligent railwayman wouldn't have understood The Railway Accident. He would think it was rather silly about something rather serious." So, in the mid-50s, he drew on the Soviet-approved style of socialist realism to embark on the three volumes of The Spiral Ascent, which tells the story of Marxist poet and teacher Alan Sebrill's efforts to reconcile his political and artistic lives.

Critic Frank Kermode says the trilogy was designed as a Marxist-Hegelian work "with the three volumes representing the theses, antitheses and synthesis. People thought the early volumes were a bit programmatic and dry, with all the passages where the characters representing Upward and Isherwood have these extra ordinarily solemn conversations about the bourgeoisie. But it gives a remarkable picture of British communism and when, in the third volume, the narrator becomes 'I', it is very personal as it reprises and transforms things from the first volume." Kermode at first thought the trilogy "was a kind of secret book that you had to go to Colletts [the left-wing bookshop] in Charing Cross Road to buy because nobody else stocked it. Reading the final volume you can see that he had in mind a new kind of novel that was more than just an exercise in socialist realism. It is a very serious and impressive attempt to combine a politics with an aesthetic."

Drabble thinks Upward suffered "from not being the vicar of Bray - he stuck to his strong convictions. But that has given his later work a sort of haunted integrity. It has a very strange note of elegy and persistence, hopelessness and hope all mixed up in a very poignant way."

"I think the British establishment has been very cunning," says Upward. "They made it clear that I was not to receive - what's that phrase? - the oxygen of publicity. But it didn't stop me going on, and in fact I made use of it." His 1994 semi-autobiographical story, "An Unmentionable Man", is a fine example of his later work with its unsettlingly fluid handling of the relationship between paranoia and repression. But for all his mistrust of the establishment he has, in one sense, been fully embraced as his papers are now at the British Library, and include letters from Auden asking for Upward's opinion on early poems. Sally Brown, head of manuscripts, says she still regularly receives new printouts of works in progress, "always with a note saying, 'destroy all previous versions'. It's our little joke because he knows very well we're not going to destroy anything." There are also over 70 volumes of diaries in his wardrobe: "He showed me the note attached to them saying 'in the event of my death these diaries are to be sent straight to Sally Brown at the British Library'."

In fact Upward's health is very good but he has grown resigned to receiving news that friends and colleagues have died. "One letter recently said that 'poor old Basil has handed in his dinner pail'," he chuckles. John Sutherland recalls visiting Upward with Spender's widow. "Upward was very interested in the details of how Stephen died. In fact it was very quick, and he must have died before he knew he was dying. Upward said, 'yes, he was always very lucky'. I've never heard anyone quite so honest about envying someone their good fortune in this respect."

Another, minor anxiety is that Upward might receive a telegram from the Queen to mark his 100th birthday next month. "I've always been a republican, but I wouldn't want to be uncivil, so perhaps I just won't let her know. And if one does turn up I'll have to find a way to let her know that she must have been in some way misinformed."

He still writes most days and says while "writing has never been a pleasure, it has been a discipline. Let it be good if possible, but at all times let it be." He says the title A Renegade in Springtime "came from an idea for a story about Auden. I never wrote the story but the phrase stayed. The renegade is the one with a sense of reality and everyone else is too happy-go-lucky. While I think Shelley's line about poets being unacknowledged legislators is probably a bit silly, I do think what writers say has an influence. In the end, I think it is something that really matters."

Edward Falaise Upward

Born: September 9 1903, Romford, Essex.

Education: Repton School; Corpus Christi, Cambridge.

Family: 1936 married Hilda Percival (died '95), one son, Christopher (died 2002), one daughter, Kathy.

Career: 1926-31 prep school teacher; '32-61 Head of English, Alleyn's School, London.

Novels: 1938 Journey to the Border; The Spiral Ascent trilogy: '62 In the Thirties, '69 The Rotten Elements, '77 No Home but the Struggle.