Inside Princess Diana's tumultuous love affair with a heart surgeon before her untimely death

— -- Princess Diana took a trip to Pakistan in 1997 that was officially presented as her doing a favor for close friends to help raise money for a cancer clinic in Lahore.

But privately, royal insiders say the trip was connected to an on-off romance she was having in London with Hasnat Khan, a British-Pakistani heart surgeon whom she called “Mr. Wonderful.”

“She was truly taken with somebody who was so pure of purpose,” said Vanity Fair journalist Sarah Ellison. “He was healing people. He wasn't remotely interested in her fame. He obviously wasn't all that interested in his own appearance. He sort of had a paunch. He ate Kentucky Fried Chicken. I mean, it's definitely this sort of aggressive kind of normalcy.”

Diana and Khan’s relationship is featured in the upcoming ABC special "The Last 100 Days of Diana," which retraces the events and details of Diana's final months through the eyes of close confidants until her untimely death in Paris 20 years ago. "The Last 100 Days of Diana" premieres Sunday, May 7, at 9 p.m. ET on ABC.

Diana was killed in a car crash in August 1997. She was 36 years old.

After her divorce from Prince Charles in 1996, Diana began a new life free from the constraints of being a royal. Diana had met Khan two years earlier in a hospital waiting room, and eventually royal insiders say they went on to have a tumultuous love affair. Those closest to her say Diane’s relationship with Khan was the most passionate of her life.

“Diana fell in love with Hasnat Khan when she went to visit a friend in hospital and there he was, the doctor with the big, limpid, dark eyes,” said Tina Brown, the author of “The Diana Chronicles.”

Royal insiders said Diana and Khan tried to conceal their relationship, spending most of their time together in private, either at his place or hers.

“She would go to his flat, his dirty flat, and do his ironing. She didn't do her own ironing but she did Hasnat's ironing,” said Diana’s butler, Paul Burrell.

Diana couldn’t hide Khan from Burrell, who said that Khan “virtually moved into Kensington Palace” during their affair.

“I would have to tell the maids, ‘No maids in the princess's rooms today,’ because she'd say to me, ‘Oh Sleeping Beauty, still asleep. He's been working all night. Leave him,’” Burrell said.

Diana’s personal chef Darren McGrady said he also knew about Diana’s secret guest.

“I remember the princess' sort of weekend menu changed slightly,” McGrady said. “She was asking for, you know, curries and things like that.”

Those close to Diana said the princess was certain Khan was “the one” for her, but he was unsure they could have a future together.

“She said, ‘I know-- I know that he will not marry me. But I don't want to lose him,’ said Diana’s friend Roberto Devorik.

Khan was a Muslim man from a traditional family, Brown said, whose “mother was going to find him the person to marry, and that’s the way it was.”

Ellison said Diana began reaching out to Khan’s family to get their support, writing to his grandmother for months in an effort to “kind of woo the family.” Royal insiders said Diana’s trip to Lahore was really about her visiting Khan’s family’s compound to get them to bless their relationship, which Brown said Khan wasn’t aware of beforehand.

“She sat there, drinking tea with his aunties and grannies and uncles and everything,” Brown said. “And they were just absolutely perplexed by this princess in their garden, sitting there having tea.

“She hadn't even warned Hasnat she was going do it,” she added. “He was, of course, absolutely outraged by it.”

Diana’s journalist friend Richard Kay said Diana’s motives in Pakistan got back to Khan and he called the situation “ridiculous.”

“I think he took her to task about this, saying, ‘What a stupid idea. We can't possibly do this,’” Kay said.

Those close to Diana said it was the princess who was pushing Khan to make their relationship public, but he didn’t want all the attention of being in the spotlight.

“I think Hasnat was very much in love with Diana,” Brown said. “But [he] had really reached the end of his tether because Diana pushed him and pushed him and pushed him to go public and say, ‘We're a couple,’ and he wouldn't. He didn't want to.”

“Hasnat said to me, ‘I don't want to be known as just Mr. Diana. I want my world. I want to be a surgeon,’” added Burrell. “And this is where their two worlds collided.”

Lady Colin Campbell, the author of “The Real Diana,” said another reason Khan didn’t want to be in a public relationship with Diana was because of his family. But despite it all, the couple continued to see each other off and on.

“The family liked her as a person,” Campbell said. “But could well see that it was a disaster waiting to happen.”

"The Last 100 Days of Diana" premieres Sunday, May 7, at 9 p.m. ET on ABC.