But he was unfaithful only after his marriage had disintegrated beyond repair, possibly as late as 1992, according to News at Ten. It said he had made his infidelity clear in the documentary about his life to be shown on Wednesday.
An ITN spokesman said the station had discovered that, in the ITV programme made to mark the 25th anniversary of the Prince's investiture, Jonathan Dimbleby asked him whether he had been faithful to the Princess.
He said their understanding was that the Prince's answer was: 'Yes.' Then he hesitated and added: 'Until it became clear that the marriage had irretrievably broken down.'
He was later asked about his feelings for a former girlfriend, Camilla Parker-Bowles, with whom he has been rumoured to have had a long- term affair. He described her as a 'dear friend', ITN claimed.
Speculation about their possible affair followed the publication of alleged tape-recordings of an intimate telephone conversation between them. Speculation about their relationship also featured in Diana: The True Story, a book by Andrew Morton which is believed to have been written with the co-operation of the Princess.
The makers of Wednesday's programme by Central Television, entitled Charles: the Private Man, the Public Role, have done their utmost to keep the contents secret after being granted unprecedented access to the Prince over more than a year. It has been portrayed by royal observers as part of a Palace 'charm offensive' to rebuild the Prince's image after his damaging separation from the Princess in 1992, 11 years after they married. It is also seen as the latest instalment in the public relations campaign being fought between the estranged couple.
A spokesman for Central last night dismissed the ITN report as speculation. But when asked if the quotes were correct, he said: 'I've no idea. We only received the film today from Jonathan Dimbleby. It's something we intend to keep under wraps until Wednesday.'
Buckingham Palace also described the report as 'speculative'. 'We will have to wait and see what is actually in the documentary,' a spokesman said .