Attenborough's "Life on Earth" in 1979 was watched by 500 million people worldwide, while dozens of subsequent documentaries and associated books have made him a household name.

"Ocean", which features spectacular footage of undersea habitats and marine life, emphasises the importance of healthy seas in tackling biodiversity and climate change.

After decades observing the natural world, Attenborough says he now understands that "the most important place on earth is not on land" and that we must "open our eyes to what is happening right now below the waves".

"We have drained the life from our oceans," he says in the film's trailer, adding: "If we save the sea, we save our world."

The documentary looks at why the oceans are in such poor health and how they can bounce "back to life", according to Attenborough.

It is not the first time the broadcaster has turned his focus on oceans.

In 2017, "Blue Planet II" became a global call to arms for environmentalists about the danger to seas, particularly from plastic pollution.

"Ocean"", produced by Silverback Films and Open Planet Studios, will be shown in cinemas in some countries from Thursday, the same day Attenborough celebrates his 99th birthday.

On the small screen, it will premiere on National Geographic on June 7 and will be available to stream globally on Disney+ and Hulu a day later.