The Prince of Wales says he will not speak out on issues when he becomes king as he "isn't stupid" and recognises that being heir to the throne and head of state are two different roles.
Prince Charles has been criticised in the past for his views on topics such as the environment or architecture, dubbed as meddling by critics.
The prince has previously used his unique position to try to influence ministers. In 2015 he had to defend his decision to write a series of letters to government ministers, some of which are known as the "black spider" memos, so-called because of his use of black ink.
A long-running legal battle by Guardian newspaper journalist Rob Evans to secure the release of the documents culminated in the UK's highest court ordering the Government to publish them.
The letters showed the prince had raised issues with ministers on several occasions between September 2004 and March 2005.
He tackled then-prime minister Tony Blair over the lack of resources for the armed forces fighting in Iraq and also wrote to ministers about the benefits of complementary medicine, the need for affordable rural homes and the threat to heritage buildings.
Interviewed for a BBC documentary about his 70th birthday on November 14, the prince acknowledged he would not be "able to do the same things I've done as heir " and as monarch would have to operate within "constitutional parameters".
In the hour-long film, the Duke of Cambridge said he would like his father to spend more time with his grandchildren - Prince George, Princess Charlotte and Prince Louis.
He said "when he's there, he's brilliant" but "we need him there as much as possible".
The Duchess of Cornwall also paid tribute to Charles's role as a grandfather, saying her five grandchildren "absolutely adore" the prince, who reads the Harry Potter books while doing the voices of the characters.
Speaking in detail about his future role as head of state the prince said: "You know, I've tried to make sure whatever I've done has been non-party political, and I think it's vital to remember there's only room for one sovereign at a time, not two.
"So, 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 - the two situations - are completely different."
Asked whether his public campaigning will go on, he added: "No, it won't. I'm not that stupid. I do realise that it is a separate exercise being sovereign. So of course I understand entirely how that should operate."
When questioned about what some have termed his "meddling", Charles defends his actions, which include establishing the Prince's Trust in 1976 to help disadvantaged young people.
He said: "But I always wonder what meddling is, I mean I always thought it was motivating but I've always been intrigued, if it's meddling to worry about the inner cities as I did 40 years ago and what was happening or not happening there.
"The conditions in which people were living. If that's meddling I'm very proud of it."
The documentary captures the future king in private and public, from feeding vegetable scraps to his chickens and collecting their eggs at his Highgrove home in Gloucestershire, to visiting Australia's Great Barrier reef to highlight climate change, and Caribbean islands recently devastated by a hurricane.
In the film, Prince, Son and Heir: Charles at 70, the royal says about his role as Prince of Wales: "You have to make of it what you feel is right.
"So, there's nothing laid down, that's what makes it so interesting, challenging and of course complicated."
The Duchess of Cornwall described in the documentary how Charles was driven by the need to help others, saying: "He feels everything inside, that's why he gets things done.
"He's pretty impatient, he wants things done by yesterday as I think everybody who works for him will tell you. But that's how he gets things done, he's driven by this, this passion inside him to really help."
She added: "He would like to save the world."
Prince, Son And Heir: Charles At 70 will be screened on Thursday at 9pm on BBC One.
