By Susanna Twidale

LONDON (Reuters) -British naturalist David Attenborough says there is hope for the future of the planet's oceans despite the scale of their damage in his new film which premiered on Tuesday evening with King Charles in attendance.

In his latest work "Ocean", Attenborough, one of the world's best-known nature broadcasters and filmmakers whose work spans seven decades, charts the challenges faced by the seas over his lifetime, from destructive industrial fishing practices to mass coral reef bleaching.

"After almost 100 years on the planet, I now understand the most important place on Earth is not on land, but at sea," he says in a trailer of the movie.

Its full release in cinemas on Thursday coincides with Attenborough's 99th birthday.

"When David Attenborough started there were two TV channels and everybody knew him as the voice of nature. Now there are hundreds of channels, social media but yet he is still the voice for nature," Enric Sala, an executive producer of the film and National Geographic Pristine Seas founder, said in an interview.

Tuesday's evening premiere in London, which was also attended by other guests including former U.S. climate envoy John Kerry and actress and model Cara Delevingne, followed a daytime screening for students and teachers which was also watched by Prince William, although he did not speak at the event.

The heir-to-the throne, like his father King Charles, has championed protecting the planet launching his multi-million Earthshot Prize in 2020, to help spur novel environmental solutions.

Attenborough has also forged a strong relationship with the royal family and is a supporter of the Earthshot prize and filmed a documentary in 2018 with the late Queen Elizabeth, "The Queen’s Green Planet".

Sala said it was important to showcase Attenborough's film to younger viewers.

"We hope that the younger generations coming to the screening today are going to feel so inspired that they will want to be the David Attenborough of the future," Sala said.

Despite depicting the bleak current state of the health of the ocean, discoveries of restored seabeds during filming offer hope.

"The ocean can recover faster than we had ever imagined, it can bounce back to life," Attenborough says in the film.

"If we save the sea we save our world. After a lifetime of filming our planet I'm sure nothing is more important."

The film's release comes ahead of the United Nations Ocean conference in June where it is hoped more countries will ratify a 2023 agreement to protect ocean biodiversity, which currently lacks sufficient signatories to come into force.

(Reporting By Susanna Twidale; Editing by Hugh Lawson and Tomasz Janowski)