"Are you Harry's father?" Princess Diana's former lover responds to the biggest Royal rumour.

The British Royal Family have endured countless rumours over the years. But perhaps the most persistent surrounds the identity of Prince Harry’s father.

Fuelled by little more than tabloid-touted speculation and a shared hair colour, it’s been suggested that Princess Diana’s former lover James Hewitt fathered the now 32-year-old.

The rumour been denied repeatedly by people close to the family, and as of last night, by Hewitt himself.

During a special episode of Sunday Night marking 20 years since the Princess’ death, the 58-year-old former army captain was questioned point-blank on the issue by Channel 7 journalist Melissa Doyle.

“Are you Harry’s father?” she said.

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“No, I’m not,” Hewitt responded.

Asked why the rumour persists, he said simply because “it sells papers”, a notion which Doyle said must be “heartbreaking” for both he and 32-year-old Prince Harry.

“It’s worse for him probably,” Hewitt said. “Poor chap.”

Prince Harry (2016) and James Hewitt (2005). Images: Getty.

Hewitt and Princess Diana were in a romantic relationship for five years after he was employed to provide her with horse riding lessons in 1986 - two years after Prince Harry was born.

The former tank commander told Sunday Night it was "so easy" to fall in love with the princess and that he has no regrets about their relationship.

"I regret some of the things that have been caused by that, but not that at all," he said. "No way."

The affair was exposed and ended in 1991, when Hewitt left for the first Gulf War.

James Hewitt during the exclusive interview with Sunday Night. Image: Channel 7.

Hewitt, who now lives in Devon with his mother, in 2003 asked for £10 million (AU$16.1m) for a collection of 64 personal letters from Princess Diana, but the sale was reportedly unsuccessful.

It's unclear if he was paid for the Sunday Night interview.

Princess Diana's divorce from Prince Charles was finalised in 1995.