The now 48-year-old heart specialist met Diana in 1995, two months before her infamous Panorama interview. The princess was visiting Joe Toffolo, the husband of her friend Oonagh, who was having triple bypass surgery at the Royal Brompton hospital supervised by Khan.
After the visit, Diana returned to the hospital almost every day for three weeks to pursue Khan and a romance eventually began.
Although it was Diana's most serious relationship since her separation from Prince Charles three years earlier, the romance was kept secret.
Diana left messages for Khan at the Brompton using the alias Dr Armani and they met in unusual places to avoid the media.
On one occasion Diana wore a black wig to slip into Ronnie Scott's, the Soho jazz club, while on another occasion Khan hid in the boot of a car to enter Kensington Palace and visit the princess, with the help of Diana's butler, Paul Burrell.
The butler was apparently asked by Diana to find a priest who would marry her to Khan, who is a Muslim.
At Kensington Palace, Diana is said to have read books on Islam and kept a photograph of "my dishy doctor".
She visited Khan's parents' home in Pakistan and introduced him to William and Harry.
But the pressure of being in the public eye became too much. Khan was convinced he would be unable to continue work as a medical professional as the formal partner of the Princess of Wales.
They considered moving to Pakistan - his preferred option - but after initially agreeing, Diana changed her mind.
The two also considered starting a life in other countries, including Australia, but they broke up in 1997, with Khan reluctant to make the relationship public, as Diana wanted.
Khan did not know about the princess's relationship with Dodi Fayed until it became public. "When I found out I was really mad; mad as hell," he said recently.
The doctor recently separated from his wife of 18 months, 29-year-old Hadia Sher Ali, the daughter of a noble Afghan family. Khan has refused to say why the arranged marriage failed, but has conceded that the events of 10 years ago continue to dominate his life.
Of Diana he revealed little, simply saying: "I found her a very normal person with great qualities.
"She did great work all over the world, not just in the UK but everywhere."
Khan has agreed that the statement he gave to the 2004 Lord Stevens inquiry into Diana's death can be read to the inquest jury.
The specialist left London in October and is staying with his elderly parents in Jhelum, Pakistan, before starting anew as the head of a cardiac hospital in Malaysia.
