Celia Fremlin used to say that she wrote the sort of book she wanted to read, in which a mysterious threat hangs over someone and escalates chapter by chapter.

Her first book, The Hours Before Dawn (1958) won the American Edgar Prize for best crime novel. It was reprinted as a Virago Modern Classic almost 40 years later, in acknowledgement not just of her ability to create an atmosphere of simmering fear and wickedness amid the trivia of daily life, but also of the meticulously observed dilemma of her heroine, a young wife so exhausted by her baby’s interminable crying that she is persuaded that her fears are psychotic. Self-deception is a recurring theme. In With No Crying (1981), the protagonist is pressured into an abortion, and then feigns a pregnancy to regain attention, with dire results.

Publishers by then sometimes treated her novels as mere precursors of the “woman in jeopardy” genre that Ruth Rendell exploited with such success (when writing as Barbara Vine). But Fremlin had an observant wit all her own, highly valued by her friends as well as her readers.

In Dangerous Thoughts (1991) the heroine is married to a man she believes is lying about his experience as a hostage. She muses on the modern belief in talking things over: “The truth is that unhappy marriages come about in large measures as the end result of a prolonged exercise in communication: in particular, the communicating of unflattering truths on a wide variety of topics, ranging from the correct handling of toothpaste to the squandering of the family fortunes on drink or self-awareness courses. In these sorts of cases, ‘Least Said Soonest Mended’ would be my proverb of choice.”

In old age Fremlin became well known for a different reason — her advocacy of voluntary euthanasia and her announcement that she had personally helped at least three friends to kill themselves. The first occasion, soon after the Second World War, concerned a woman friend with motor neuron disease, and the barbiturates were supplied by her husband, Dr Elia Goller. She remembered that with triumph but was haunted by the time when an assisted suicide went wrong — something that she had imagined with prescient horror in one of her short stories, A Lovely Day to Die (1984).

Celia Margaret Fremlin was born in 1914 in Ryarsh, Kent, and educated at Berkhamsted School for Girls and Somerville College, Oxford, where she read classics and philosophy. During the Second World War she worked in London for Tom Hopkinson’s Mass Observation project, interviewing people in streets and air-raid shelters before and after bombs had dropped. She was proud of this, considering it much better than modern market research. “We used to write down what people actually said, but with these ‘yes’, ‘no’, ‘not very much’ replies you miss the flavour of their words.”

She and Goller were married in 1942 and had a son and two daughters. In one terrible month in 1968 her daughter Sylvia died in an accident and Goller suffered a heart attack — and, rather than live a debilitated life, took an overdose.

For several years she could not write. In her sixties she began to take long walks at night all over London, from Brixton to Hackney, alone. Talking as she had during the Blitz to hundreds of people, mostly women, she recorded many strange or sad stories, but never came across a single first-hand account of injury or attack. She published her findings in New Society in 1979, to the astonishment of younger readers. Her conclusion was that, to make the dark streets lose their terror, “We don’t need more policemen on the beat. We need more grandmothers.”

Her second marriage to Leslie Minchin, a translator of German Lieder and gas consultant, restored her confidence as a writer, and several of her later novels — she wrote 15, plus dozens of short stories — were gratefully dedicated to him: she often said she hoped they might contrive to die together, but he died in 1999.

Fremlin was predeceased by all three of her children.

Celia Fremlin, mystery writer and voluntary euthanasia activist, was born on June 20, 1914. She died on June 16, 2009, aged 94