One of the Arctic 30 Greenpeace activists held in jail in Russia has been released on bail.

In a move welcomed by the environmental group, pre-trial bail was granted by a court to a Russian woman who was among 30 activists accused by the authorities of hooliganism.

Yekaterina Zaspa, 37, was ruled not to be one of the activists who tried to scale an Russian arctic rig being used to tap oil reserves near the North Pole. She was the doctor on board the Arctic Sunrise vessel used by the protesters.

Zaspa and 29 others have been behind bars in Russian detention centres since 24 September for their action - which prosecutors claimed was piracy.

Greenpeace vowed to continue efforts to get the remaining activists released from prison.

John Sauven, executive director of Greenpeace UK, said: "There is no rational reason to release one activist on bail and keep another in prison for three months. The Russian courts are making decisions that have a huge impact on the detainees and their families in what appears to be a completely arbitrary manner.

"We will be appealing against all decisions denying bail to our activists and the two journalists imprisoned with them."

The court process was branded a "high farce" by Mads Christensen of Greenpeace International. He claimed that an Australian man was confined on the grounds that he was a flight risk, despite having no passport.

"They should all be released from prison," said Christensen. "This is a scandal, this bears no relation to the administration of justice. We will do everything we can to get our people out."

Sir Paul McCartney called on Russian president Vladimir Putin to release the environmentalists from jail.