Sir David Attenborough is launching what he says is one of the most important films of his career as he enters his hundredth year.

He believes his new, cinema-length film Ocean could play a decisive role in saving biodiversity and protecting the planet from climate change.

Sir David, who will be 99 on Thursday, says: “After almost 100 years on the planet, I now understand the most important place on Earth is not on land, but at sea.”

The ocean is the planet’s support system and humanity’s greatest ally against climate catastrophe, the film argues. It shows how the world’s oceans are at a crossroads.

The blue carpet was rolled out for the premiere the Royal Festival Hall in London.

In attendance was the King, told Sir David he “can’t believe” his 99th birthday was on Thursday.

The pair posed for photos before they entered the auditorium for the screening.

A host of celebrities also attended, including singer Geri Halliwell-Horner, astronaut Tim Peake, singer James Blunt and model Cara Delevingne.

Earlier, Prince William attended a matinee in a “private capacity”, along with hundreds of children from schools across the country.

Toby Nowlan, who produced Ocean, says this new production is not a typical Attenborough film. “This is not about seeing brand new natural history behaviours. It is the greatest message he’s ever told,” he says.

The film documents how the state of the world’s oceans and our understanding of how they function have changed in the course of Sir David’s lifetime.

Sir David remembers his first scuba dive on the Great Barrier Reef way back in 1957: “I was so taken aback by the spectacle before me I forgot – momentarily – to breathe.”

Since then, there has been a catastrophic decline in life in the world’s oceans. “We are almost out of time,” he warns.

Ocean contains some of the most graphic footage of the damage that bottom trawling – a common fishing practice around the world – can do to the seabed. It is a vivid example of how industrial fishing can drain the life from the world’s oceans, Sir David claims.

The new footage shows how the chain that the trawlers drag behind them scours the seafloor, forcing the creatures it disturbs into the net behind. They are often seeking a single species: more than three-quarters of what they catch may be discarded.

“It’s hard to imagine a more wasteful way to catch fish,” comments Sir David.

The process also releases vast amounts of carbon dioxide which contributes to the warming of our planet, yet bottom trawling is not just legal but is actively encouraged by many governments.

Sir David says the state of the ocean has almost made him lose hope for the future of life on the planet. What has kept him from despair is what he calls the “most remarkable discovery of all” – that the ocean can “recover faster than we had ever imagined”.

Sir David says the story of the world’s whales has been a source of huge optimism for him.

It is estimated that 2.9 million whales were killed by the whaling industry in the 20th Century alone. Scientists have said it is the largest cull of any animal in history when measured in terms of total biomass. It pushed almost all whale species to the edge of extinction.

Just one per cent of Blue Whales were left, recalls Sir David: “I remember thinking that was it. There was no coming back, we had lost the great whales.”

But in 1986 lawmakers bowed to public pressure and banned commercial whaling worldwide. The whale population has rapidly recovered since then.

One of the film’s directors, Keith Scholey, has worked with Sir David for 44 years. “When I first met David, I was in shorts,” he jokes. That was in 1981, two years after Sir David had resigned as the BBC’s director of programmes – one of the most senior jobs at the Corporation. “He’d done one career, and he was off on his next.”

Despite now nearing his 99th birthday Sir David is still remarkably energetic, says Scholey. “Every time you work with David, you learn something new,” he says. “It’s really good fun. But also, David keeps you on your mettle, because he is so on his mettle and so, you know, it’s always a very creative process.”

Sir David’s key message in the Ocean film is that all is not lost. Countries have promised to protect a third of the world’s oceans. He hopes his new film will spur leaders to take firm action on this promise at a UN conference next month.

He believes that could be transformational.

“The ocean can bounce back to life,” Sir David says. “If left alone it may not just recover but thrive beyond anything anyone alive has ever seen.”

A healthier ocean ecosystem would also be able to trap more carbon dioxide, helping protect the world from climate change, according to scientists.

“In front of us is a chance to protect our climate, our food, our home,” Sir David says.

As he celebrates his 99th birthday this week he is still fighting to protect the natural world he has worked his lifetime to show to us in all its glory.

Ocean will be in cinemas across the country from Thursday.