Media playback is unsupported on your device Media caption Prince Charles says it would be "nonsense" to think he could campaign publicly as king

The Prince of Wales says he will stop speaking out on topics he feels strongly about when he becomes king, as he is "not that stupid".

He has campaigned on issues such as the environment for decades but says he would not do the same as monarch.

Speaking in a BBC documentary to mark his 70th birthday, Prince Charles said the idea that he would continue making interventions was "nonsense".

He said he would have to operate within "constitutional parameters".

What has he said?

In the documentary, Prince Charles was asked about his campaigning, which some people have described as "meddling".

Documentary film-maker John Bridcut, who followed the royal for 12 months, said Prince Charles "bridled a bit" at the use of the word "meddling" and instead preferred to think of his interventions as "motivating".

The heir to the throne said: "If it's meddling to worry about the inner cities as I did 40 years ago, then if that's meddling I'm proud of it."

Image copyright Getty Images Image caption The Duchess of Cornwall said Prince Charles had a passion to help

He said the roles of Prince of Wales and king were completely separate.

"It's vital to remember there's only room for one sovereign at a time, not two," he said.

"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 would continue, he said: "No, it won't. I'm not that stupid."

BBC royal correspondent Nicholas Witchell said, however, that, in private, Prince Charles could still seek to influence government policy.

He said: "The definition of what he is entitled to do as sovereign is to encourage and to warn."

Why is this significant?

By Nicholas Witchell, BBC royal correspondent

He has spent his adult life trying, as he puts it, "to make a difference". Often that has led the Prince of Wales to speak out about topics about which he feels deeply: the environment, GM crops, inner cities, architecture, education, homeopathic medicine and others.

It has, on occasions, caused irritation within the government departments that have had to respond to his heartfelt "black spider" handwritten letters raising, always courteously but often insistently, some issue that has come to the prince's attention.

It has all given rise to a greater concern. Does Prince Charles fully appreciate that, when he succeeds his mother and becomes Britain's king, these interventions will have to stop?

Image copyright Jonathan Brady Image caption Prince Charles alongside his grandson Prince George

Those who know him have said for years that privately he fully understands that, as king, he would have to stop his "campaigning".

Prince Charles himself has always baulked at saying as much publicly. He's said to feel that any reference to how he will function as monarch could be seen as being disrespectful to his mother.

However, with the Queen now in her 93rd year, and with Prince Charles about to celebrate his 70th birthday, he has finally said - publicly and explicitly - that he does recognise his interventions on matters of public debate will have to stop as soon as he becomes king.

"You operate," he says in the BBC documentary, "within the constitutional parameters."

It is reasonable to suppose his assurances will be heard with some relief within Whitehall and the corridors of power.

What has he campaigned on?

Architecture

In 1984, Prince Charles gave a speech criticising a modernist design for an extension to the National Gallery in London.

He said it looked like a "vast municipal fire station" and, if it were to be built - the proposal was subsequently dropped - would be a "monstrous carbuncle on the face of a much loved and elegant friend".

Indeed, the Prince of Wales has campaigned for traditional urban design and architecture for many years.

And this led to heir to the throne establishing the Prince of Wales's Institute of Architecture in 1986, now known as The Prince's Foundation.

In 1992, he said: "I didn't particularly want to see this country, which I mind about and love greatly, disappear under a welter of ugliness."

Climate change

The heir to the throne has spoken passionately about climate change and environmental sustainability.

In 2017, he said humans must live in harmony with their environment.

He added: "Failure to do so, and I fear that we are already perilously close to doing this, will prove catastrophic."

BBC environment analyst Roger Harrabin said: "The prince warned decades ago about human-induced climate change.

"It was controversial to some at the time but now there's scientific consensus on the threat."

Wildlife conservation

Prince Charles has been a staunch supporter of wildlife conservation and anti-poaching campaigns.

Image copyright Getty Images

In 2018, he said: "With the current rate of biodiversity loss now running at 1,000 times the natural rate, we appear to have managed to initiate the planet's sixth great extinction event and still seem utterly hell bent on destroying what little we have left."

Homeopathy

Prince Charles has sparked some criticism for his support of homeopathy - a treatment based on the use of highly diluted substances to allegedly aid the body to heal itself.

In 2015, he told an international gathering of scientists and government officials he treated his own cows and sheep with homeopathy.

Clarence House later said: "Homeopathy is used on a case-by-case basis at Home Farm, in combination with more conventional medicine, to minimise dependence on antibiotics."