Within a period of six years, Ted Hughes faced the sudden deaths of four people dear to him. In February 1963 his estranged wife, Sylvia Plath, gassed herself in her kitchen following his affair with another woman, Assia Wevill. He was just 32 when he found himself in sole charge of their children, Frieda, who was three, and Nicholas, barely one year old.

Six years later, in March 1969, Wevill killed herself and Shura, their four-year-old daughter. At that time, his mother Edith appeared to be getting on well after an operation on her knee, but Hughes was afraid that the news might affect her recovery. In the following weeks he shunned his parents, and did not visit, phone or write to them. When his father asked Olwyn, Hughes's sister, what the matter was, she told him but made him vow to keep it a secret. But he could not keep silent and told his wife. Edith suffered a thrombosis, lapsed into a coma and died three days later. Ted was certain that Wevill's suicide was the final blow.

In a letter to his close friend Lucas Myers, Hughes reflected on his part in the deaths of his wife and lover, confessing that with Plath it was his "insane decisions", while in Wevill's case it was his "insane indecisions". When he granted us a rare interview in London in October 1996, Hughes said Plath's death "was complicated and inevitable, she had been on that track most of her life. But Wevill's was avoidable." Perhaps this was why he tried to erase her from his life.

The press refrained, for some mysterious reason, from reporting the tragedy. The crime columns of the London newspapers that week in March 1969 ran items about the strangling of a wife in her home and the death of a girl who set fire to herself in Paris, but there was no word of the deaths of Assia and Shura Wevill in Clapham Common. Only one local paper, the South London Press, violated what amounted to a hush-up. Even there, the story was at the bottom of page 13, and omitted any hint of an intimate connection between the poet and the deceased.

Throughout his life, Hughes warded off biographers and journalists and asked his friends to refrain from mentioning him in interviews or in their memoirs. When his archive at Emory University in Atlanta, Georgia, was made available to the public in 2000, it was devoid of Wevill's presence in his life: none of the numerous letters they exchanged, the notes, drawings or photos, were there.

In the rare instances where he agreed to provide biographical details, Assia and Shura were never mentioned. He claimed that after Plath's suicide and until his marriage to Carol Orchard in 1970, he raised his children assisted only by members of his family or a local woman who helped with the daily chores - that for all those years, he was looking for a permanent feminine figure but "the right woman failed to materialise". In fact, Wevill lived with him in what she called Plath's "ghost house", 23 Fitzroy Road, London, and then in Ireland and Devon, and mothered his children.

In May 1962, Assia and her third husband, the Canadian poet David Wevill, were invited to spend a weekend with Plath and Hughes, who were then living in the village of North Tawton in Devon. It was on that weekend, as Hughes later wrote in a poem, that "The dreamer in me fell in love with her". Six weeks passed before he and Wevill met alone for the first time, when he came to London for a meeting at the BBC.

But Plath was quick to discover the budding affair. She ordered him out, and he was happy to comply. The following day he knocked on the Wevill's door carrying four bottles of champagne. Wevill made no secret of Hughes' ferocious lovemaking among her office friends. Equally repelled and fascinated, she told Edward Lucie-Smith, "You know, in bed he smells like a butcher." In the next two months he shuttled between the two women.

In mid-September he and Plath took a holiday in Ireland. On the fourth day he disappeared. His whereabouts have remained a mystery not only to Plath but to subsequent biographers and scholars. However, in our research we discovered that when Hughes embarked on the Irish trip, he already had a ticket to another destination. Ditching Plath in Ireland, he hurried to London to meet Wevill, and the two of them headed south for a 10-day fiesta in Spain. He and Plath had spent their honeymoon there, and she hated the country. For him and Wevill, the trip was a delight, providing them with a creative boost: a film script that they had started writing together.

When he returned home, Hughes had a terrible row with Plath; he refused to give up his mistress and left for London permanently. Two months later, Plath moved to London as well. Hughes and Wevill were no longer making a secret of their affair. They were seen everywhere, so much so that many people mistakenly thought that they were actually living together.

On February 11 1963, Plath ended her life. Two days later, Myers came for a condolence visit and found Wevill resting in Plath's bed. A month later Hughes and Wevill decided to abort the child that Wevill was carrying.

Wevill and her husband, David, agreed to a six-month trial separation, and he went to Spain, while she moved in with Hughes and his children. They spent the summer vacation with Hughes's parents in Yorkshire. In her diary, Wevill described a family dinner: as she was leaning over the table to give Frieda her food, she and Hughes collided in a kiss.

In September 1963 Hughes returned with his children to Devon, leaving Wevill on her own in the death-flat: he wanted some time to himself and she had serious doubts about his commitment to her. When David returned from Spain, he joined Wevill in Plath's flat.

The affair continued, but they lowered its profile and invented a secret code for their correspondence. Hughes dispatched daily love letters to Wevill's home, addressed to F Wall Esq. It was a private joke between the lovers, that Hughes was the fly on the wall at the Wevills'. It is puzzling as to why they went to all this trouble when they could have used Wevill's office address, as they had done before: was it for the thrill, and the romantic intrigue? Alexandra Tatiana Elise (nicknamed Shura) was born on 3 March 1965. Ten months later, Wevill left David and moved with Ted to Ireland. It was a time of bliss, and Hughes was delighted and relieved that his children had taken to their half-sister.

Hughes was exploding with ideas. He even found the strength to probe into the suicide of his wife and began writing Crow, handing Wevill the drafts to comment on. Later, he dedicated the book to her and Shura. Wevill's diaries, still in private hands, give a unique account of Hughes at work, "like a great beast, looking over an enormous feast, dazzled and confused by the variety".

Wevill was a perfect partner: artistically inclined, she wrote some poetry and was a rather skilful painter, but she was not ambitious and had no artistic ego herself. "He's almost incapable of performing one word wrong," she wrote admiringly. She said she felt reverence in the company of "one of God's best creations".

Together, they were working on a book of Hughes's poems and her drawings. The theme was A Full House, a pack of cards in which the kings, queens and knaves were biblical, mythological and historical figures.

The poet and translator Michael Hamburger, Hughes' good friend, remembers another project of Wevill and Hughes: Wevill showed him "miniature paintings in brilliant colours with many animals and plants". Neither project materialised, and though some of the poems survived none of Wevill's drawings has.

Originally, they planned to stay in Ireland for five years, but after a few months they had to return to Devon, to tend to Hughes's ailing mother. His parents were dismayed by their son's scandalous relationship with the haughty, thrice-married woman: they feared it had ruined his reputation. Hughes's father determinedly ignored Wevill's presence. He never spoke to her, refused to sit at the same table and averted his eyes when she put a plate of food in front of him. And in any disagreement between Wevill and his parents, Ted invariably sided with them.

Edith Hughes was moving from one bout of illness to another, and eventually Hughes realised that she would never get better as long as she and Wevill lived under the same roof. He initiated a disengagement plan: Wevill and Shura would return to London and wait there until his mother was strong enough to return to her home in Yorkshire. On odd weekends Wevill and Shura came down to Devon, and he visited them in his trips to London. They continued looking for a house of their own, but Hughes found fault with them all. It dawned on Wevill that being at his side at the time of Plath's suicide had contaminated her for ever, and that he would never marry her. "I have lived on the dream of living with Ted - and this has gone kaput," she wrote in her suicide note to her father. "There could never be another man. Never."

Shura had become the core of Wevill's existence, and she was quite certain that if left motherless, the four-year-old, pampered child would be a second-class citizen in the Hughes household. She was afraid that Shura was too old to be adopted, and did not wish her to grow up alone as a foster child, an orphan. Her murderous act was thus the outcome of a distorted over-responsibility: "Execute yourself and your little self efficiently," Wevill had written in her diary three days before.

Soon after her death, Hughes wrote a poem in which he tormented himself about having been destructive towards his nearest and dearest "who were my life". He never published it. Was it because it contrasted with the account that he wished to leave for posterity? In 1990, he published a volume of 20 poems, Capriccio, which revolved around Wevill. In it, he blamed her for consciously burning herself on Plath's funeral pyre.

· A Lover of Unreason: The Biography of Assia Wevill by Yehuda Koren and Eilat Negev. Published by Robson Books, price £20.00. To order a copy for £18.00 with free UK p&p go to theguardian.com/bookshop or call 0870 836 0875