Princess Diana was so deeply in love with Pakistani heart surgeon Hasnat Khan that she kept a copy of the Koran by her bedside and even contemplated moving to Pakistan with him, a newspaper reported on Saturday.
And she began her affair with Dodi al Fayed, who was killed in a car crash along with her in August 1997, only to make Khan jealous, the Daily Mail said.
"Diana studied the Koran and kept a copy by her bedside. And she discussed what life would be like for her in Pakistan with her friend Jemima Khan, who had married the country's cricketing hero Imran Khan - a marriage that was later to break up," the paper reported.
It said Diana suddenly plunged into an affair with Dodi, son of the Egyptian-born owner of Harrods Mohamed al Fayed, after Hasnat Khan told her the only way for them to have a successful married life was if they moved to Pakistan.
But Khan also explained to her that he was not convinced she would be able to come to terms with life in Pakistan.
"From this state of doom, suddenly she was having fun and her friends were divided as to what was behind the new relationship," the newspaper said.
It added that while some saw it as an affair on the rebound, others thought she was getting back at her husband Prince Charles for giving his long-time lover Camilla Parker Bowles a widely publicised 50th birthday party.
"However, the majority were convinced she was seeing another Muslim to make Dr Khan jealous in the hope of winning him back," it said.
The paper quoted a friend of Khan as saying he felt responsible for her death "because, but for him, she wouldn't have been with Dodi in the first place".
Khan is now married to a distant cousin of his, Hadia Sher Ali, a descendant of Afghan royalty, and is unwilling to talk about his past, the paper said.
The report comes amid heightened media interest in the life and loves of Princess Diana as a public inquest continues into her death at the behest of Mohamed al Fayed who alleges a conspiracy to kill Diana.
Khan has been summoned by the coroner to give evidence at the inquest but is said to be undecided if there is any point in appearing before what the Daily Mail called "the circus that has unfolded at the High Court".