A former White House intern is speaking out about an 18-month affair she had with US President John F. Kennedy while he was in office, revealing intimate details in a new book to be published on Wednesday.  

Mimi Alford, who was 19 when she started her internship at the White House in 1962, discussed her secret sexual affair with the president in her book "Once Upon a Secret: My Affair with President John F. Kennedy."  

She told NBC's Meredith Vieira that she decided to write the book to stop her secret becoming "deadly."

"It's not so much that I feel that I'm exposing myself, it's that I'm really unburdening myself - it was a very difficult thing for me to do," Alford said in an interview to be broadcast on Wednesday.

The former intern discussed details including swimming with the president just four days into her internship, and doing her homework en route to Washington to have sex with Kennedy.

"I should have felt guilty. He was married to Mrs Kennedy. But I didn't at the time -feel guilty," Alford said.

There have been numerous reports since his 1963 assassination of Kennedy having had extra-marital affairs with several other women, including allegations of a dalliance with Marilyn Monroe.

"I think that's probably partly what makes me feel sad when I look back. It made me feel very special," Alford said of reports of Kennedy's other affairs.

The full interview with Vieira will air on NBC's "Rock Center with Brian Williams" on Wednesday.