The Prince of Wales has sought to quell concerns that he would be a “meddling” or activist king , saying “I’m not that stupid”. His words, the strongest yet on the subject of his kinghood, are to be broadcast in a documentary on BBC One on Thursday to mark his 70th birthday.

Declaring the role of sovereign and heir to be different, Prince Charles maintains he is aware of the constraints he will encounter as king, and says his behaviour will change. Acknowledging that some people have accused him of being a meddling prince, he argues that he has always tried to be non-party political. “It is vital to remember there’s only room for one sovereign at a time. Not two,” he says.

He also says in the documentary, entitled Prince, Son and Heir, Charles at 70: “You can’t be the same as the sovereign if you’re the Prince of Wales or the heir. But the idea somehow that I’m going to go on in exactly the same way if I have to succeed, is complete nonsense because the two situations are completely different.

“You only have to look at Shakespeare plays, Henry V or Henry IV part I and 2, to see the change that can take place. Because if you become the sovereign then you play the role in the way that it is expected.

Facebook Twitter Pinterest Copy of the letter Charles wrote to the prime minister Tony Blair, to influence agricultural policies. Photograph: Philip Toscano/PA

“So, clearly I won’t be able to do the same things I’ve done as heir. So, of course, you operate within the constitutional parameters. But it’s a different function. I think people have forgotten that the two are very different.”

Charles has faced criticism for decades over his campaigning on issues such as GM crops, architecture, integrated medicine, and climate change. When told people have expressed worries that this would continue in the same way, Charles responds: “No. It won’t. I’m not that stupid. I do realise that it is a separate exercise being sovereign. So, of course, you know, I understand entirely how that should operate.”

Asked if he could continue, as king, to use his “convening power”, he replies: “Well, you never know. But you could only do it with the agreement of ministers. That’s how it works.”

The Duchess of Cornwall says that Charles knows“his destiny” so, she says: “I don’t think it does weigh on his shoulders at all.” It was not something her husband talked about very much, she says. “It’s just something that’s going to happen.”

The Duke of Cambridge implores his father to spend more time with his grandchildren, George, five, Princess Charlotte, three, and Prince Louis, six months. Appealing to Charles to slow down, he says: “I would like him to have more time with the children.” Prince William adds: “Having more time with him at home would be lovely, and being able to play around with the grandchildren. Because when he’s there, he’s brilliant. But we need him there as much as possible.”

Prince, Son and Heir, Charles at 70, on BBC 1 at 21.00 on Thursday 8 November.