The Prince of Wales has described the challenge of protecting the world's oceans as "like none that we have ever faced before."

In a speech delivered to the Blue Economy and Finance Forum in Monaco, Prince William said life on the ocean floor was "diminishing before our eyes" and called for ambitious action "on a global, national and local level".

At the Grimaldi Forum, named after Monaco's royal family, the Prince spoke in both English and French as he laid out what was at risk.

"The truth is that healthy oceans are essential to all life on earth. They generate half of the world's oxygen, regulate our climate and provide food for more than three billion people," he said.

Rising temperatures, pollution and overfishing are causing huge damage to the world's oceans and the communities that rely on them.

The forum comes ahead of the UN Ocean Conference in Nice, France, this week, with the events looking at the role oceans play in global trade, food security and sustainable energy.

In Monaco on Sunday, the Prince was speaking to an audience of environmentalists, scientists and investors – many of whom have travelled there with a view to financing ocean protection projects.

Prince William acknowledged that investing in ocean work can be a tricky proposition for investors.

"All too often, it can feel distant and disconnected from our everyday lives, allowing us to forget just how vital it is," he said. "We must realise the potential of the blue economy for our ecosystems, our economies and our communities."

The Prince was speaking as founder of the Earthshot Prize, which gives out five £1m prizes each year for the best solutions to the greatest climate challenges.

Several Earthshot winners and past finalists were in the audience.

Enric Sala, of the National Geographic Society's Pristine Seas project, was a finalist in 2021 and has pioneered work to protect marine life.

He is also part of the team that has produced Sir David Attenborough's new film, Oceans, which Prince William described as "the most compelling argument for immediate action I have ever seen".

"Watching human activity reduce beautiful sea forests to barren deserts at the base of our oceans is heartbreaking," the Prince said.

"For many, it is an urgent wake up call to just what is going on in our oceans. But it can no longer be a matter of 'out of sight, out of mind'."

He ended his speech saying action was needed for future generations and quoted Sir David.

"If we save the sea, we save our world."

The Prince interviewed Sir David at the premiere of Oceans last month, with the film described by its producer as "the greatest message [Sir David] has ever told".

Kensington Palace described the speech as a "landmark intervention" by Prince William, using his platform to generate change and bring in investments to scale up ocean solutions.

While in southern France, the Prince met President Chavez of Costa Rica, France's President Macron and Prince Albert of Monaco - a supporter of many oceans projects and a key player at the forum.

Prince William will also attend a closed session, held in private, with ocean experts and investors.