On 8 June 1924, Andrew Irvine, a 22-year-old Oxford rowing blue, disappeared with George Mallory as they made their final attempt on the summit of Everest. For decades historians have focused on the better-known Mallory, with little interest in the student who died with him, except to ask why Mallory should have chosen to climb Everest with him in the first place.

Volumes have been written about Mallory - 'the Galahad of Everest' - his life, his disappearance and his obsession with climbing the world's highest mountain. Now, 86 years on, a biography of Irvine written by his grand-niece, Julie Summers, is about to redress the balance. It reveals for the first time the undisclosed turmoil in the young man's life - including the affair with his best friend's stepmother - which contributed to his decision to go to Everest to escape the scandal.

For while Irvine was struggling towards the top of Everest, his lover, a former chorus girl named Marjory Thomson, was being divorced by her husband, the steel magnate Harry Summers - Julie Summers's great-grandfather - over their affair.

Irvine had first met Marjory through her stepson Dick, a shy, dark-haired boy whom Irvine had befriended on the fives court at Shrewsbury School.

The relationship was close. Summers had been devastated by the premature death of his natural mother and thrived on his friendship with Irvine. He gave Irvine the nickname 'Sandy', for his shock of fair hair, by which he would be known. As the friendship deepened, Irvine, who grew up in Birkenhead, would spend summers with Summers at the luxurious Cornist Hall in North Wales. Dick later married Irvine's sister Evelyn, Julie Summers's grandmother.

If Harry Summers's wealth impressed the young Irvine, it also impressed 19-year-old Marjory, whose blue eyes and 'charming, sunny personality' completely bowled over Summers, then 52. They were married in 1917, but the marriage would quickly founder as Marjory lost interest in her older husband and soon started to seek out company more her own age.

By now Irvine had gone up to Oxford in 1922, shoe-horned into Merton for his prodigious talent as a rower, having emerged as one of the stars of the 1919 'Peace Regatta' at Henley. He was part of the winning Oxford eight in the 1923 Boat Race.

In the aftermath of a war that had decimated a generation, it is easy to forget the impact of these young sporting heroes - on young women in particular. 'They were almost like gods,' one female spectator wrote of the Oxford crew, men who had just missed serving on the Western front. 'We stood and stared in awe and admiration.'

And although she had first known him as a boy, Sandy's impressive physique, charm and new celebrity made a considerable impact on Marjory Summers. She began an indiscreet affair, driving Irvine to the theatre in her husband's Rolls and taking him for intimate picnics in North Wales. When Irvine was invited on an Oxford University expedition to Spitsbergen, Marjory joined the team for the first leg to Tromsø, Irvine visiting her first-class cabin at night.

Harry Summers became aware of his wife's infidelity when a friend saw Irvine leaving her bedroom in the middle of the night, soon after he returned from Norway. Divorce proceedings were instigated by the millionaire.

Summers speculates that Irvine loved Marjory but was relieved to be out of the relationship, considering Marjory too poor a catch for marriage.

Irvine's other great relationship, with George Mallory, was perhaps the most unlikely of all. Mallory was 38 and hugely experienced. Irvine had barely climbed at all, but had impressed Noel Odell, who had joined the Spitsbergen expedition as geologist. Irvine was practical, able to strip and repair machinery; that and his strength were, to the Mount Everest Committee, more use on Everest than climbing prowess alone.

During the voyage to India, Mallory, desperate to succeed, found his young partner likeable if dull-witted, telling his wife Ruth that Irvine was 'one to rely on, for everything except for conversation'.

Fearless on Everest: The Quest for Sandy Irvine, by Julie Summers, Weidenfeld & Nicolson, £20.