The King has said he is standing up for the environment on behalf of younger generations and his grandchildren, as he gives hope that combating climate change “is still possible”. Featuring in a new documentary with adventurer Steve Backshall, Charles speaks of his frustrations about inaction towards climate change and explains that he wants to avoid leaving “a ghastly legacy of horror” that younger people will have to deal with.

In a new feature-length documentary that will air as part of ITV’s Christmas coverage later this month, the 77-year-old monarch shows the public a different side to him. His knowledge and passion for the environment shine through as always, but viewers also get a fresh insight into his extreme sense of adventure and dry sense of humour.

The producers hope that the monarch will be able to reach a demographic that climate campaigners cannot reach, as they aim to tell crucial stories while also entertaining and engaging a mass audience.

The film follows Steve Backshall as he retraces the King’s steps from his 1975 trip to the Canadian Arctic exactly 50 years on.

Aged just 26, then Prince Charles embarked on a royal tour like no other, braving the freezing climates of the Arctic and hardcore challenges such as a 30-minute under-ice scuba dive and driving a skidoo at a rapid 50mph.

Meeting at Buckingham Palace before the Deadly60 presenter travelled to the Arctic, the King tells Mr Backshall: “I am riveted that you want to do this, 50 years later.”

Asked what he remembers from the expedition, Charles immediately responds: “Well, diving under the ice. That I vividly remember.”

The pair chat as they look through archive images from the King’s trip, including pictures from his own personal collection that have never been shared with the public before.

When asked if the monarch feels nostalgic about the trip, he says: “Yes, I think I am too old to go back. But yes, absolutely. So, thank God, I was younger in those days. I could never have survived it now.”

Aides had only scheduled 10 minutes for the King to sit down with the much-loved wildlife presenter, but their shared enthusiasm for conservation and protecting the planet meant that the meeting lasted over half an hour.

A source described that they struck up “a tremendously warm rapport and found common ground on a huge variety of issues”.

A small crew of one cameraman, soundman and the director filmed the show over two weeks, showcasing the Arctic’s natural beauty and the challenges it now faces.

The King, who was extremely supportive of the project, tells Mr Backshall that he was glad he saw the frozen frontier “as it should be”, as the Arctic is undergoing rapid and alarming changes due to climate change, warming up to four times faster than the global average.

Melting ice and thawing permafrost are significantly disrupting the ecosystem and threatening wildlife like polar bears and seals. The wildlife presenter heads to Coronation Glacier, showing that the terminus has retreated almost 1.5km in the last half-century.

“The only place that climate change becomes an absolute reality is a glacier like this,” Mr Backshall says. “Where I am standing right now within a decade or so all of this ice is going to be gone. And this mighty glacier, one of the most beautiful places you could ever see is being affected by our world, our life, our choices. Scientists warn we ignore these melting glaciers at our peril.”

The King, who believes that everything in nature is interconnected, tells Mr Backshall that indigenous peoples have a crucial role to play in helping to ensure the planet’s future. “Their knowledge is absolutely crucial now for helping to rescue the situation,” he says.

Shown archive footage of himself speeding along the snow in a skidoo at 50mph, the King said: “It was quite exciting. There is no point in going too slowly.” Mr Backshall then tried to match the monarch’s speed, but didn’t even manage to break 40mph.

Perhaps the most impressive of the then-prince’s achievements on his 1975 trip was his ice dive, which saw him plunge into a hole cut in the sea ice.

Shocked that his security detail allowed him to dive under one metre of ice, the King joked: “I am afraid I have always tended to live life dangerously.”

Footage then shows Charles in a bright red diving suit under the water, as he reflected: “It was a fascinating experience, I wouldn’t have missed it for anything.”

Joe Macinnis, the dive master who accompanied the King below the ice, has kept the unisuit all this time and revealed that Charles had a bit of fun while in the freezing water.

When it was time to resurface, the monarch rose to the top holding an umbrella and a bowler hat. “I thought it would be rather fun coming back up with the hat on,” Charles told then presenter. “And I thought – a chance to foll around.”

In archive footage, the King told an interviewer it was “bloody cold”, and prompted laughter when he expanded his suit to as big as it would go.

Mr Backshall wraps up the programme by saying that while the trip highlights a major cause for concert, he also “found a great determination to ensure this stunning land and ultimately the planet itself has a health future”.

As an environmental champion for over five decades, the King reiterates this and says: “We have to believe that it is still possible. Because you have to have hope. But to provide that hope you have to take the action. I mean, all I can do is behind the scenes. But I mean it – it can get very frustrating, to say the least.”

He continues: “I mind for what it’s worth about the younger generations. To me it is not fair to leave them something in a far worse state than I found it, if you know what I mean. The whole point I have always felt is to improve it for people, so they don’t have a ghastly legacy of horror to have to deal with. That’s why I spent all these years, because I don’t want to be accused by my grandchildren of not doing anything about it. That is the key.”

The King had a sneak preview of the documentary earlier in the year and is understood to have been pleased with the final edit.

Steve Backshall’s Royal Arctic Challenge will air on Thursday December 18 at 8.30pm on ITV1.