A group of Greenpeace activists have climbed aboard two oil platforms in the North Sea to protest against "unacceptable" decommissioning plans.
Campaigners from the Netherlands, Germany and Denmark say they have occupied the closed Brent Alpha and Bravo rigs in a protest aimed at oil and gas giants Shell.
They say four of Shell's Brent platforms will be left abandoned with their storage tanks containing more than 11,000 tonnes of oil.
Banners have been unfurled, reading: "Shell, clean up your mess" and "Stop Ocean Pollution".
Joris Thijssen, director of Greenpeace Netherlands, said: "Shell's plans are outrageous and go against international agreements to protect the sea.
"The 11,000 tons of oil that is still stored in the foundation of the platforms will sooner or later end up in the sea. That is unacceptable.
"The North Sea is not a garbage dump, Shell has to clean up its mess."
Mr Thijssen urged nations adhering to the OSPAR Commission, set up to protect the marine environments of the North-East Atlantic, to act.
He added: "Greenpeace urges all OSPAR governments to protect the sea and not to give in to the pressure of a major polluter Shell."
Campaigner Dr Christian Bussau said Shell's plans to leave parts of the four Brent platforms at sea were a "scandal".
He said: "Shell's plans are a scandal and go against international agreements to protect the environment.
"With escalating climate emergency, biodiversity loss and species extinction, we need healthy oceans more than ever.
"Abandoning thousands of tonnes of oil in ageing concrete will sooner or later pollute the sea. Shell must be stopped."
Greenpeace International said that although a ban on dumping installations and platforms in the North East Atlantic ocean was agreed in 1998 Shell has requested an exemption from the UK Government.
It called for governments to protect the ocean and "not cave in to corporate pressure".
Dr Bussau said Shell's "reckless business model" threatened some of the world's most important ecosystems with extinction.
He added: "Shell is directly fuelling the climate emergency that is causing more extreme storms, floods, droughts and wildfires and bringing misery to millions of people around the world.
"The company's reckless business threatens some of the world's most important ecosystems with extinction and has to be stopped. For us to have a future, toxic oil companies like Shell must have no future."
But a Shell spokesman said the company had spent 10 years conducting in-depth research into decommissioning the Brent platforms and its recommendations were the result of more than 300 scientific and technical studies.
He added: "We can confirm that two protesters have boarded the Brent Alpha platform and one has climbed onto the Brent Bravo concrete legs.
"Their safety and that of our workers are our prime concern at this moment.
"Our proposals were submitted only when we were convinced they were the best option: safe, environmentally sound, technically achievable, and socially responsible."
Earlier this year a group of Greenpeace activists staged a similar protest on board a 27,000-tonne-rig as it attempted to leave the Cromarty Firth.
Protesters boarded the BP platform in June to call for an end to drilling for new wells in the Vorlich field, which is believed to hold up to 30 million barrels' worth of oil.