King Charles, Queen Camilla and the Duchess of Edinburgh are in attendance for the premiere of the monarch's documentary Finding Harmony: A King's Vision.
The screening has been held at Windsor Castle in Berkshire, with several stars joining the royals for the production.
Kate Winslet, who narrates the documentary, posed for the camera, as did Beneditch Cumberbatch alongside his wife Sophie Hunter.
Dame Judi Dench, Alan Titchmarsh and Sir Rod Stewart are also in attendance for the production's premiere.
The King and Queen attended His Majesty's documentary premiere | PA
Sophie wore a long, green dress for the screening
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Other celebrities who journeyed to Windsor include Formula One star Sebastian Vettel and actor Sir Kenneth Branagh.
The documentary, set to be released on Prime Video next month, focuses on the King's environmental campaign, despite years of fierce criticism.
It looks back at the King's philosophy and lifelong commitment to green issues.
The 90-minute film shows His Majesty urging people to take a more serious approach to the climate, with mankind "actually destroying our means to survival".
Sir Rod pictured before the screening got underway
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Beneditch Cumberbatch pictured alongside his wife Sophie Hunter
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He hopes there will be greater awareness of the issue, with a "need to bring things back together again".
The King, in new footage, explained: "I just felt this was the approach that I was going to stick to. A course I set, and I wasn't going to be diverted from."
Charles previously warned that the approach to climate change is "rapidly going backwards," as the monarch shared his concerns.
He said: "People don't seem to understand it's not just climate that's the problem - it's also biodiversity loss, so we're actually destroying our means of survival, all the time."
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Dame Judi Dench was part of the A-listers at the event
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Sir Alan Titchmarsh also attended the premiere
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The King continued: "To put that back together again is possible, but we should have been doing it long ago. We've got to do it as fast as we can now.
"Maybe, by the time I shuffle off this mortal coil, there might be a little more awareness… of the need to bring things back together again."
Archival footage of the King, showing him talking to plants, is included in the film.
Ian Skelly, co-author of the King's 2010 Harmony book, said: "Those criticisms really upset him. He got treated very unfairly, seen very unfairly, and those of us that knew him better were quite upset by that.
Kate Winslet narrated the King's documentary
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"It was difficult to know how to respond, but I really felt for him."
A spokesman for the King said the film was "not a conventional royal documentary".
"There are no golden carriages here; no glittering crowns or crimson robes.
"Instead, this is a deeply personal exploration of ideas that have shaped His Majesty's life and work: the interconnectedness of all things, the wisdom of traditional knowledge, and the belief that we can build a future that works in partnership with nature rather than against it," the spokesman said.