Prince Charles has promised to tone down his public comments on environmental and other issues if he succeeds his 92-year-old mother, Queen Elizabeth II, as Britain's monarch.

Speaking in a BBC documentary to be aired later Thursday, Charles dismissed as "nonsense" fears that he could continue to intervene in issues he cares about if he becomes king.

Asked about concerns that he had been "meddling" by speaking out over several decades on issues such as the environment, urban development and wildlife protection, Charles said he had tried to be "non-party political" and would accept the "constitutional parameters" of a monarch.

"I think it's vital to remember there's only room for one sovereign at a time, not two," he said in the one-hour programme ahead of his 70th birthday next Wednesday.

"So, you can't be the same as the sovereign if you're the Prince of Wales or the heir," Charles added, using one of his official titles.

"But the idea, somehow, that I'm going to go on in exactly the same way, if I have to succeed [the queen], is complete nonsense because the two situations are completely different."

Asked whether his public campaigning would continue, Charles told the broadcaster: "No, it won't. I'm not that stupid."

In an earlier clip from the programme, Prince Harry, the youngest of Charles' two sons, said that when he and his brother, Prince William, were children their father had made them pick up litter to show the importance of environmental protection.