Kate Winslet will narrate King Charles' documentary about saving the planet

She said the documentary willl offer a "fascinating insight into the King's work as an environmentalist".

She added: "It is a both pleasure and a privilege to be a part of this film. I share His Majesty's passion for protecting our planet and building sustainable communities, so it's been really rewarding to work with The King's Foundation on this exciting project.

"I know audiences will learn, laugh and feel inspired by what's featured in the film, and I hope the impact of Harmony will be felt in years to come."

The king previously admitted his "fondest hope" is that the project will encourage a new generation to help save the planet.

He said: "Nature is our sustainer; we are a part of nature.

"Therefore, what we do to her, we do to ourselves. For much of my life, I have sought to promote and encourage ways we can work with, rather than against, nature. In other words, to restore balance to our planet, which is under such stress.

"This film will, I hope, demonstrate just some of the remarkable work being done around the world to put harmony into practice, from the forests of Guyana to sustainable communities in India - and, closer to home, through the work of my King's Foundation at Dumfries House and Highgrove.

"Never has it been more important for the world to make a concerted effort to protect and prioritise our planet and to restore our relationship with it.

"It is therefore my fondest hope that this film may encourage a new audience to learn about the philosophy of harmony - and perhaps inspire the same sense of determination it has given me to help build a more sustainable future."

Filming began in January at Dumfries House in Ayrshire, where The King's Foundation is based, and guests in the documentary will include the charity's ambassador Sir David Beckham.

Kristina Murrin, chief ex ecutive of the King's Foundation, said: "At The King's Foundation we are proud to have stood beside the King for the past 35 years of his 'harmony' journey and it is both moving and inspiring to see that journey committed to film.

"Our work spans many of the King's passions, whether that be protecting traditional skills, teaching young people about nature and the outdoors, or building green towns and cities with sustainability at their core. It is gratifying to see this work represented on screen in Finding Harmony."

Cameras have also been present at the 76-year-old monarch's Highgrove home, and in July they captured footage of his first 'Harmony Summit', which saw the monarch draped in a circlet of feathers and a scarf while sacred smoke coiled up over the apple trees in a nod to traditional cultures and forms of healing.

The documentary will see the king looking back over archive footage of his speeches and engagements but his appearances are said to be "minimal" so the film can focus on his work and ideas.