Her husband, Hunter Davies, said she died Monday at a hospice in London. The couple spent much of their time in England's peaceful Lake District and also in London.
Davies, also an author, said she underwent a double mastectomy 40 years ago and recovered fully, but the cancer returned to her back 10 years ago.
"I thought she was a goner," he said of her initial bout with cancer.
"It was bad, but eventually she recovered. At age 50 on her birthday, she got up at 5 (am) and ran up Red Pike mountain and then swam in the lake and came back to the house and brought me tea," he said.
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The Royal Society of Literature issued a statement describing her as "an extraordinarily prolific and gifted writer of fiction, non-fiction and literary criticism."
"Georgy Girl" was made into a hit movie in 1966 starring Lynn Redgrave, Charlotte Rampling, Alan Bates and James Mason.
Forster co-wrote the screenplay for the film, which also inspired a bouncy pop tune that captured the exuberance of the times.
Forster taught at a girls' school in north London before she started to write books.