The UK risks being shut out of a historic oceans summit because parliament has failed to ratify the UN’s high seas treaty, environmental charities and campaigners have warned.

The high seas treaty, formally known as the Agreement on the Conservation and Sustainable Use of Marine Biological Diversity of Areas Beyond National Jurisdiction, comes into force on Saturday, after two decades of talks.

But although the UK has signed the treaty, the bill to ratify it – introduced last September – still needs to pass through parliament and be deposited at the UN.

In a letter to the foreign secretary, the heads of 18 UK environmental charities have condemned “the glacial pace of government progress” on ratifying the treaty, which was officially adopted in 2023. They call for the bill to be passed in good time before the first Ocean Cop summit, which is expected to take place this year, to ensure Britain is included.

“Disappointingly, while the world celebrates, the UK is still not among the 81 countries, including China, France, Japan, Spain, Mexico, Brazil, that have signed the treaty into law,” the letter says. “We urge the government to complete it at least 30 days before the first Ocean Cop, which could take place as early as August this year, in order to guarantee the UK’s attendance there. It would be a failure of leadership to miss it.”

The biodiversity beyond national jurisdiction bill is due for its third reading in the House of Lords on Monday. The Foreign Office has been contacted for comment.

The high seas form part of a global commons that covers nearly half of the planet. However, as they lie beyond national borders, there has until now been no legal framework to protect biodiversity in them.

“The entry into force of the high seas treaty marks a long-awaited turning point for ocean governance,” said Dr Lance Morgan, the chief executive of the Marine Conservation Institute. “For decades, these vast waters – and the abundance of biodiversity that inhabits them – were beyond the reach of effective protection. Now, the global community has both the mandate and the responsibility to act.”

The new treaty creates, for the first time, the tools to create marine protected areas on the high seas, and sets clear obligations on how to ensure ocean resources are used sustainably.

In St Leonards-on-Sea, East Sussex, a striking new mural will be unveiled this weekend to celebrate the treaty coming into force. The colourful artwork, twice the length of a doubledecker bus, was painted on the town’s beachside Bathing Hut cafe as part of a global action in which artists, Indigenous peoples, activists and communities from 13 countries across five continents, representing every ocean, worked with Greenpeace to create ocean protection-inspired street art to mark this significant moment.

A Foreign Office spokesperson said: “The UK government is committed to make this into law, using the standard democratic parliamentary process. This treaty was signed under the last government and this government is committed to ratifying this quickly.”