New York - John F Kennedy carried on an 18-month-long affair with a teenaged White House intern, according to a new book published this week by a 69-year-old grandmother who claims to be the late US president's lover.

Excerpts of the shocking memoir, Once Upon a Secret: My Affair with President John F. Kennedy and Its Aftermath, were released Monday by the New York Post, which said it had purchased a copy of the book at a local bookstore.

In it, author Mimi Alford wrote a tell-all account detailing how at the time their relationship started in the summer of 1962, she was less than half the age of the dashing president, assassinated the following year at age 46.

In an excerpt published by The Post, Alford wrote that she met Kennedy just four days into her internship, and that he invited her the following day on a personal tour of the White House residence that included first lady Jackie Kennedy's bedroom.

Fifty years on, Alford - now a 69-year-old grandmother and retired New York City church administrator - writes that she lost her virginity to Kennedy that day.

“Slowly, he unbuttoned the top of my shirtdress and touched my breasts,” Alford - at the time Mimi Beardsley - wrote in the excerpt.

“Then he reached up between my legs and started to pull off my underwear. I finished unbuttoning my shirtdress and let it fall off my shoulders.”

“After he finished, he hitched up his pants and smiled at me” before pointing her in the direction of the bathroom, the Post reported.

“I was in shock,” Alford wrote.

“He, on the other hand, was matter-of-fact, and acted as if what had just occurred was the most natural thing in the world.”

The young woman, described by one Kennedy biographer as a “tall, slender, beautiful” college sophomore, continued the relationship for a year-and-a-half - even travelling with the president on occasion - until their affair ended with Kennedy's assassination.

Kennedy is said to have carried on numerous White House affairs during his presidency, including with an alleged dalliance with Hollywood starlet Marilyn Monroe.

The Post wrote that Random House, which published the book, says that Alford after the president's death “grieved in private, locked her secret away and tried to start her life anew, only to find that her past would cast a long shadow - and ultimately destroy her relationship with the man she married.” - Sapa-AFP