Previously unseen and unpublished letters between Sylvia Plath and her therapist Dr Barnhouse have uncovered a much darker side to her famous, if toxic, relationship with fellow poet Ted Hughes.

In a letter dated 22 September 1962, the date that the couple separated, Plath alleges that Hughes had physically abused her shortly before she miscarried their second child in 1961. Plath also claimed in another letter dated a month later that Hughes had told her that he wished she was dead. The letters also detail Plath’s feelings about Hughes’s infidelity with their neighbour Assia Wevill in July 1962.

Feminist scholar Harriet Rosenstein collected these letters as part of an archive while researching an unfinished biography of Plath. Put up for sale by an antiquarian bookseller for $875,000 (£695,000), the archive also includes Plath’s medical records from 1954, some correspondence with other friends and interviews with Barnhouse discussing Plath’s therapy sessions. 

Scholars had been unable to piece together the events of Plath’s life prior to this discovery, due to the fact that Hughes had destroyed a huge amount of them, allegedly to protect their children Freida and Nicholas. These letters give insight into Plath’s inner thoughts from 18 February 1960 to 4 February 1963, incidentally the week before her death. 

The literary match met as students at Cambridge University in 1956, married within four months and inspired each other to write the most memorable works of both of their careers: Plath’s being The Bell Jar and Hughe’s being his Hawk in the Rain poetry collection. Public fascination has surrounded their relationship and its disintegration, with Hughes attempting to have the final word on the matter with his poem Last Letter, published in 2010.

Barnhouse inspired the character of Dr Nolan in Plath’s autobiographical novel The Bell Jar. When Plath moved to England she stopped her treatment with Barnhouse for her mental health, but maintained a close relationship and correspondence.

The majority of Plath’s most famous poetry collection, Ariel, was written in October 1962, during the period that these letters cover. Co-editor Peter Steinberg has suggested that Plath may have found “catharsis in writing to Dr Barnhouse, and that in doing it freed her to write those explosive, lasting poems.”

The first two volumes of Plath’s collected letters are due to be published on 5 October by Faber. But the future of the letters are uncertain, as Smiths College, Plath’s alma mater, filed a lawsuit last month claiming that they are part of the Barnhouse estate.