Lawyers for Greenpeace International have demanded immediate access to 30 of activists detained by armed Russian authorities who boarded the environmental group's ship Arctic Sunrise last Thursday.
The call came as the Arctic Sunrise was towed into the vicinity of Murmansk port in western Russia.
There are reports Russia intends to prosecute all 30 activists but Greenpeace said it had not received any formal contact from the authorities.
The Foreign Office confirmed that six British nationals were being detained by the Russian authorities. British officials had been in touch with the six activists and were offering consular assistance.
The mother of one of the Britons said she was worried by reports he could be charged with piracy.
Sue Turner, 62, said she has yet to hear from engineer Iain Rogers, 37. "If they charge them with piracy there is a 15-year jail sentence and a £10,000 fine. I'm not so concerned by the fine, we can get around that. But Russian prisons are not the nicest places to be."
Russian investigators have told reporters they will file piracy charges against an unspecified number of activists who tried to climb on to an offshore drilling platform owned by state-controlled gas company Gazprom.