Simone Signoret, 64, one of France's most celebrated actresses and the winner of an Academy Award for her role as the rejected older woman in the 1958 British film "Room at the Top," died of cancer yesterday at her country home at Eure, west of Paris.

Miss Signoret was also a gifted writer whose novel, "Adieu, Volodia," a story of two Jewish immigrant families in France from 1921 to 1944, was published earlier this year and immediately became a best seller in France.

With her husband, actor-singer Yves Montand, she was a political activist whose espousal of left-wing causes and widely publicized visits to Eastern European nations during the Cold War years of the 1950s provoked extensive media commentary. She never became a communist, she insisted, and in recent years her views had shifted toward the center as she turned her efforts toward issues involving human rights abuses and racism.

In a telegram to Montand yesterday, French President Francois Mitterand said Miss Signoret "spoke for more than 40 years to the hearts of the French people."

Jack Lang, the French minister of culture, called her an "unshakeable militant favoring human rights in all regimes and on all horizons . . . . "

Miss Signoret once described herself as having a face that bore on it the scars of the laughter, tears, questions, amazement and certitudes of all her friends, gathered in a lifetime that began when she was born in 1921 in Weisbaden, Germany, the daughter of a Polish Jew on occupation duty in Germany after the cataclysm of World War I.

She grew up in a suburb of Paris, survived the bitterness of the German occupation in World War II, in which her father fled to England, and rose to prominence during the turbulence of postwar France.

Miss Signoret was known for her throaty, husky voice, and she was recognized as one of Europe's more sensuous actresses. She had few rivals when playing the part of a woman in love: she conveyed passion with looks and gestures that were both subtle and explicit.

She appeared in more than 40 films. In her early roles she often played prostitutes or kept women. As she aged she put on weight and those roles changed to bourgeoise housewives and frumpy older women. But she always retained at least a suggestion of sensuality and lasciviousness. Her career spanned an era in which French cinema came into its own.

Filmgoers in the United States knew her best for her role in "Room at the Top," the frank, raw British film about love, sex and ambition in which Miss Signoret was the rejected older woman opposite the late Laurence Harvey. She was the first actress to win a major Academy Award without ever having made a Hollywood film.

Initially the part had called for a Yorkshire housewife, but no British actress was thought to have sufficient sensuality for it, and it went to Miss Signoret, whose notices subsequently were credited with helping the film achieve its immense worldwide popularity. In 1977 Miss Signoret played an aging prostitute in "Madame Rosa," which won an Academy Award for best foreign film.

Born Simone Kaminker, Miss Signoret took her mother's maiden name to avoid the Nazi persecution of the Jews during the German occupation. She studied classics and literature in secondary school, worked briefly as a typist during the war but was soon persuaded by friends that she could earn more money as a film extra.

Her first film appearance was in "Le Pt, Robert T. and James, both of Port Tobacco, Md., and Alfred E., of Port Orange, Fla.