The marriage of John Kennedy Jnr and his wife had descended into a relationship rocked by drug abuse, violence and the prospect of imminent separation when they died in an aircraft crash, a new biography claims.</p>Just two days before the son of the assassinated US president John F Kennedy died in the 1999 crash, he phoned a friend and revealed that his marriage to Carolyn Bessette was deteriorating and that his wife's drug habit and her recourse to violence was making life impossible.</p>"It's got to stop. Otherwise we're headed for divorce," Mr Kennedy told his friend, according to the work by family biographer Edward Klein. At the time Kennedy made the call he was staying at the Stanhope Hotel in New York, where he had gone to escape his wife.</p>He added: "I want to have kids, but whenever I raise the subject with Carolyn she turns away and refuses to have sex with me."</p>Kennedy, 38, his wife and his sister-in-law, Lauren Bessette, were killed in July 1999, when the aircraft he was flying crashed into the sea off Martha's Vineyard, Massachusetts.</p>Mr Klein's book, excerpts of which were published yesterday in Vanity Fair</i> magazine, said that by the time they died the couple were essentially living separate lives. He quoted Mrs Kennedy's fashion industry friends as saying she was known to be a heavy drug user.</p>A staff member from Kennedy's political magazine, George,</i> told of a dinner at which Mrs Kennedy made at least a half dozen trips to the bathroom, returning to the table with white rings around her nostrils.</p>"We went from bar to bar, and she wanted to come over to my apartment, but I said no, because I knew it would be an all-nighter," the friend said. "I dropped her off at 3am."</p>The next day, Kennedy asked why his wife had been out so late. The staff member replied: "A better question is why your wife didn't want to go home."</p>The biography also claims that Kennedy believed his wife was continuing an affair with an old flame, Michael Bergin, a Baywatch</i> actor and former underwear model. </p>