NEW YORK (Reuters) - A long time ago, and in a galaxy far, far away, actress Carrie Fisher had an affair with her "Star Wars" co-star Harrison Ford, she said.

Fisher spilled her 40-year secret to People magazine this week while promoting her memoir, "The Princess Diarist," which goes on sale on Tuesday. The book is based on Fisher's diaries from the time and is billed as telling behind-the-scene stories of the first "Star Wars" movie.

Fisher said the three-month affair started, and ended, during the filming in 1976 of "Star Wars: A New Hope," in which she played Princess Leia to Ford's romantic, maverick hero Han Solo.

"It was so intense," Fisher, now 60, told People on Tuesday. “It was Han and Leia during the week, and Carrie and Harrison during the weekend."

Harrison, who at the time was married to his first wife, Mary Marquadt, did not respond to requests for comment. Fisher said the "Indiana Jones" actor, who is known for guarding his personal life, had received an advance draft of her book.

Fisher and Ford reprised their roles last year for the new blockbuster movie "Star Wars: The Force Awakens."

Back in 1976, Fisher said the pair spent their first night together following a birthday party for "Star Wars" creator and director George Lucas. She was 19, and Ford was 33.

"I looked over at Harrison," she wrote in the memoir, according to excerpts published by People. "A hero’s face — a few strands of hair fell over his noble, slightly furrowed brow.

"How could you ask such a shining specimen of a man to be satisfied with the likes of me? I was so inexperienced, but I trusted something about him. He was kind."

Fisher said she had never spoken of the affair for 40 years but felt the time for secrecy was over.

"Harrison’s very private, but I think I waited an appropriate amount of time," she told People. "How much longer could I wait?"

(Reporting by Jill Serjeant; Editing by Lisa Von Ahn)