Greenpeace repeats call to allow activists and journalist to go home after it emerged Russia's Investigative Committee wrote to one of the 30 saying they are not allowed to leave the country

The Russian authorities have told a group of Greenpeace activists and freelance journalists arrested after a protest against oil drilling in the Arctic that they cannot leave the country.

The environmental group said the decision was in defiance of a ruling of an international court, and repeated its demand that the 28 activists and two journalists, including six Britons, should be allowed home.

The so-called Arctic 30 were arrested in September after the Russian authorities boarded their vessel during their protest.

They have all been granted bail by courts in St Petersburg but have remained in Russia while efforts are made to give them permission to leave.

Greenpeace revealed on Friday that Russia's Investigative Committee has written to one of the 30 – Anne Mie Jensen from Denmark – indicating that they are not free to leave the country.

Lawyers for Greenpeace said they expect all of the non-Russian defendants will be treated in the same way by the authorities, meaning they would now be forced to stay in St Petersburg for Christmas and possibly well beyond.

Lawyers have also been seeking an assurance that the investigative committee would give at least one month's notice when it wanted to interview the 30; otherwise they could break their bail conditions if they returned home.

In its letter to Anne Mie, the committee said it would not provide the requested notice.

Peter Willcox, the American-born captain of the Greenpeace vessel, the Arctic Sunrise, said: "I am ready to go home to my family. We were seized in international waters and brought to Russia against our will, then charged with a crime we didn't commit and kept in jail for two months.

"A respected international court says we should be allowed to go home, so do numerous presidents and prime ministers, but we can't get visas to leave the country, and, even if we could, there's no guarantee the investigative committee won't schedule an interview for the day I get home, forcing me to break my bail conditions.

This is either a mistake and we're caught in a vicious bureaucratic circle, or it's a deliberate snub against international law. Either way this is a farce."

A ruling in November by the International Tribunal for the Law of the Sea, made up of 21 eminent judges, ordered Russia to allow the Arctic 30 to leave the country immediately and to release the Arctic Sunrise, as soon as a bond of 3.6 million euros (£3 million) in the form of a bank guarantee was paid.

The bond was posted by the Government of the Netherlands, where the Arctic Sunrise is registered, on 29 November, so Greenpeace said Russia was now in defiance of that order.

Greenpeace International legal counsel Daniel Simons said: "The Russian Federation is now in clear breach of a binding order of an international tribunal. As president Vladimir Putin stated in his famous open letter to the American people on Syria, 'The law is still the law, and we must follow it whether we like it or not.'

"In his state of the nation speech in Moscow yesterday, he added 'We try not to lecture anyone but promote international law'. It's time for the authorities to act in that spirit and allow the Arctic 30 to go home to their families immediately."

Greenpeace added that an amnesty decree likely to be voted on by the Duma - the Russian parliament - this month could still see legal proceedings against the Arctic 30 dropped.

A draft of the decree submitted by President Putin does not include the Arctic 30, but the group said a small amendment by the Duma would see them covered by the amnesty.