Russian authorities have been told they must release the Greenpeace ship used in a protest against an Arctic drilling platform.
The Arctic Sunrise was seized and its crew arrested after the incident in September, with many of the activists having been released on bail this week.
Now the International Tribunal for the Law of the Sea has ordered Russia to release the ship and crew.
Tribunal president Judge Shunji Yanai of Japan said an application from Dutch authorities for the release of the ship and crew had been accepted by the panel by 19 votes to two.
Russian officials are to study the ruling, but said the tribunal had no jurisdiction over its prosecution of the activists..
“The Arctic Sunrise ship was used as a tool to commit acts that are unacceptable under international law and Russian legislation,” the country’s foreign ministry said in a statement.
Meanwhile two more Britons arrested by Russian authorities during a Greenpeace protest in the Arctic have been freed on bail after spending two months in detention.
Anthony Perrett, from Newport in South Wales, and freelance journalist Kieron Bryan are the first of the six Britons who are part of the so-called Arctic 30 to leave detention.
The move followed court hearings in St Petersburg.
Mr Bryan said it had been “tough” for the group, who were arrested two months ago.
“More than anything, it’s the isolation and not being able to speak to anyone, not being able to speak a common language. Being trapped in a cell for 24 hours a day is something I don’t ever want to experience again, and there are a lot of people inside who have to carry on doing that.
“My plan now is to go to a hotel and have a long shower, leave my room whenever I want, never do another sudoku puzzle again, and speak to my family. That’s the main plan.
“No idea about what happens next. I’m just focusing on getting through today, speaking to my family, and I’ll deal with everything from then.”
The 30 were arrested during a protest against drilling in the Arctic and were initially held in Murmansk before being moved to St Petersburg.
They were charged with piracy but the authorities said this would be changed to hooliganism.
Greenpeace said the 30 had done nothing wrong, and the group has launched a worldwide campaign to have them freed.
The six Britons were among 28 activists and two freelance journalists arrested by Russian security forces.
Greenpeace International Arctic campaigner Ben Ayliffe said: “This will only really be over when he and the others are able to go home.
“For now, we are allowing ourselves a sigh of relief, all the time remembering that those brave men and women are still charged with a crime they didn’t commit, and of course our friend Colin was refused bail. This saga is far from over.
“The detainees who were released yesterday slept well last night, though some of them stayed up late talking and talking, sharing experiences. They’ve been enjoying the taste of good food and the feel of thick mattresses, but they keep talking about Colin, asking when he’ll be out, and they don’t yet know if they can go back home or if they’ll have to stay in St Petersburg.”
Australian Colin Russell, a radio engineer, was refused bail earlier this week.