New diaries reveal the 'dark secrets' of Siegfried Sassoon's swooning affair

In it, the man famous for his 1917 counterblast against the Great War and the writer known for his moving anti-war poems and the novel Memoirs of a Fox Hunting Man, is portrayed as one half of one of the most extraordinary literary passions of the 20th century.

Sassoon's homosexuality - he referred to it as his "dark secret" - has long been known, as is the love affair he conducted with artist and cross-dressing socialite Stephen Tennant. But Max Egremont's biography of the poet, published this week, details for the first time the uninhibited and - 40 years before homosexuality was declared legal - dangerous affair.

Egremont's coup was to be not only the first person to obtain access to Sassoon's diaries for the years after 1926, but also to Tennant's personal journals. The result, drawing on extracts from these diaries, is detailed accounts of everything from the couple's bizarre driving trips to Europe (usually with a pet parrot and nanny) to their intimate sexual relations.

Mr Egremont quotes Sassoon's own depiction of the first night he spent with Tennant, whom the poet describes as "the most enchanting creature I have ever met". The two drove about 12 miles from a party at Tennant's Wiltshire house to Stonehenge, and stayed out until dawn, Tennant making "the most passionate avowals and simply intoxicating my senses".

As the relationship develops, Sassoon describes taking another midnight stroll with Tennant, who wears only a silk dressing gown. Sassoon writes: "I can't control him (or myself) on such occasions." For his part, Tennant says that the famous poet often left him "swooning with happiness". One passage in his own journal reads: "He put his mouth over mine crushing it - some kisses seem to draw the very soul out of one's body - his do mine. I feel all my heart swooning at the touch of his mouth - my soul dies a hundred million deaths when his face is on my face and neck."

Sassoon, by then in his forties, had already made a considerable name as the writer of "The Hero", "On Passing the New Menin Gate" and "Suicide in the Trenches", which concludes:

"You smug-faced crowds with kindling eye

Who cheer when soldier lads march by,

Sneak home and pray you'll never know

The hell where youth and laughter go."

Peter Parker, a critic and biographer specialising in the literature of this period, said the book demonstrated the full extent to which Sassoon was a man driven by his sexual passions.

"This has given us a far broader understanding of the man," he said. "He was obviously sexually repressed, which could well have helped drive him. This gives further depth to our knowledge of him as an extremely complicated character, a recklessly brave officer, and a pacifist, torn between the worlds of the English gentry and the life he shared with his gay lover in London."

Mr Egremont said: "These diaries cast an extraordinary light on the relationship between Sassoon and Tennant. They are a very detailed description of a gay love affair. You have to remember that homosexuality was illegal in this country until 1967."

The passionate, fiery romance began after Sassoon and the outlandishly camp Tennant were introduced by mutual friends, and lasted from 1926 to 1932. A year later, Sassoon married his wife Hester. Tennant was a glamourous figure, photographed by Cecil Beaton and used as the model for Cedric Hampton by Nancy Mitford in her novel Love in a Cold Climate. He later became a recluse and worked for 40 years on his novel Lascar.

His great-nephew, the playwright Simon Blow, said he was overjoyed that the contents of both diaries were finally become public knowledge. "I'm very glad that the record is being set straight, and I think this will be a very important biography," he said. "Some people have seen Uncle Stephen as a shallow person, but he wasn't at all. These diaries tell us so much more, and paint a far more detailed picture of this wonderful relationship between him and Siegfried.

With the help of the poet's only son, George, Mr Egremont gained access to the Sassoon diaries, hitherto jealously guarded by the Sassoon estate. No biography of Sassoon appeareduntil the late 1990s, a fact bemoaned by scholars for decades.

Peter Parker said the decision of Sassoon's son to release the diaries was "immensely brave", given the graphic detail. "Quite a lot of what is revealed is very explicit. Here is a man with a complicated homosexual private life," he said. "It is one thing to release that if you are talking about a distant relative, but quite another if it is your own father."