"Covering more than two-thirds of the ocean, the agreement sets binding rules to conserve and sustainably use marine biodiversity," United Nations Secretary-General Antonio Guterres said.

Decades of overfishing, pollution from shipping and warming oceans from climate change have damaged life below the surface.

In the latest assessment of marine species, nearly 10% were found to be at risk of extinction, according to the International Union for Conservation of Nature (IUCN).

Three years ago countries agreed that 30% of the world's national and international waters - high seas - must be protected by 2030 to help depleted marine life recover.

But protecting the high seas is challenging. No one country controls these waters and all nations have a right to ship and fish there.

Currently just 1% of the high seas are protected, leaving marine life at risk from overexploitation.

So, in 2023 countries signed the High Seas Treaty pledging to put 30% of these waters into Marine Protected Areas.

But it was only able to enter force if more than 60 nations ratified it - meaning they agreed to be legally bound by it.

With many nations requiring parliament approval, ratification can often take more than five years, Elizabeth Wilson, senior director for environmental policy at environmental NGO The Pews Charitable Trust, told the BBC at the UN Oceans Conference earlier this year. She said this was "record time".

The UK introduced its bill for ratification to Parliament earlier this month.

Kirsten Schuijt, director general of WWF International, hailed "a monumental achievement for ocean conservation" after the treaty threshold was reached.

She added: "The High Seas Treaty will be a positive catalyst for collaboration across international waters and agreements and is a turning point for two-thirds of the world's ocean that lie beyond national jurisdiction."

Mads Christensen, executive director of Greenpeace International, called it "a landmark moment" and "proof that countries can come together to protect our blue planet".

"The era of exploitation and destruction must end. Our oceans can't wait and neither can we," he added.

Once the treaty comes into force, countries will propose areas to be protected, and these will then be voted on by the countries that sign up to the treaty.

Critics point out that countries will conduct their own environmental impact assessments (EIA) and make the final decision - although other countries can register concerns with the monitoring bodies.

The ocean is crucial for the survival of all organisms on the planet. It is the largest ecosystem, is estimated to contribute $2.5 trillion to world economies, and provides up to 80% of the oxygen we breathe.

Key oceans treaty crosses critical threshold to come into force