The tragic actress had threatened to go public with her affairs with John F. Kennedy and his brother Bobby, who was America's Attorney General at the time, if her lover didn't divorce his wife and marry her - and Jackie stepped in to make a personal plea to Monroe during a tense meeting at New York's Carlyle Hotel, according to biographer Darwin Porter's new book Marilyn: Rainbow's End.
In an extract, published in America's Globe magazine, the author writes, "Jackie begged Marilyn not to publicly humiliate her children in front of the world. She also pleaded with Marilyn not to make John, Jr. and Caroline the victims of a divorce."
Porter claims Monroe was touched by her heart-to-heart with Jackie and agreed to keep her romances with the President and his brother a secret, but the biographer alleges the actress had one more big surprise for Kennedy on the day before her death in August, 1962 - she called to tell the President she had aborted his baby.
Hours later, Monroe was found dead in her Los Angeles home. Her passing was ruled a suicide, but Porter has unearthed evidence that suggests the star was murdered by Chicago Mob boss Sam Giancana for refusing to help Mafia bosses poison Cuban leader Fidel Castro, who the author claims idolised the actress.
In Rainbow's End, the biographer also claims Monroe had lesbian trysts with Joan Crawford, Marlene Dietrich and Elizabeth Taylor, who she allegedly bedded after a Frank Sinatra party in 1961.
Porter also suggests Monroe turned down a marriage offer from her one-time husband Joe DiMaggio's son, romanced Elvis Presley and Sinatra and was the star of a collection of sex tapes secretly filmed by Mafia bosses to ruin her.