James Hewitt, much criticised over memoirs to be published next week, as well as interviews he has conducted in the past, also said their relationship brought "happiness to a very unhappy woman".

Diana: Sent Hewitt 64 letters

His comments came in an interview with ITN's Trevor McDonald, to be broadcast on the ITV2 digital terrestrial television station on 23 October.

The former cavalry officer, now 40, said he first spoke to journalists in 1993 "at the behest of Diana herself", in an attempt to disguise the depth of their relationship.

"She urged me to give a description of a relationship that wasn't necessarily the whole truth," he said. "I did that and it backfired."

Hewitt has been accused of trying to cash in on 64 love letters the Princess sent him between 1989 and 1991, the contents of which are expected to be hinted at in the memoirs.

Memoirs "won't upset William and Harry"

But he denied that he wrote the autobiography for money. "I needed to do it to tell the truth once and for all," he said.

He also denied reports that Diana had tried to get the letters back.

Asked if he thought publishing his book might upset Princes William and Harry, Major Hewitt said: "Publishing the truth - that is the important thing."

The former Life Guards officer said the affair had been "in the knowledge of many people long before it became public knowledge.

"If those in authority had wanted it to end they would have been able to do so. They allowed it for a number of reasons and one of the reasons was that it brought happiness to a very unhappy woman".

Just one regret

He said the relationship "petered out" in 1992.

"I had to go to Germany when she still needed a lot of help and support back in England and I wasn't able to give it to her any more."

Asked whether he regretted anything that had happened, Major Hewitt said he regretted the affair was in the public domain.

And questioned on whether he regretted that Diana felt he had let her down, he said: "In a relationship two people have arguments. It doesn't mean to say that you must regret ever having met or having had a relationship."

Last month, the Mail on Sunday pulled out of a deal to serialise the autobiography, after criticisms that Major Hewitt would profit by featuring the letters sent to him by Diana.