David Cameron is urging Vladimir Putin to make sure that 30 Greenpeace activists imprisoned in Russia can come home.
The British Prime Minister has called the charges against the activists "excessive" and has urged the Russian President to "de-escalate" the situation.
The 30 activists, including New Zealanders David Haussmann and Jonathan Beauchamp, have been charged with hooliganism by the Russian authorities after their ship, the Arctic Sunrise, was seized.
They were initially charged with piracy, although it was later changed to hooliganism, which carries a lesser sentence.
Cameron spoke to Putin this week and told him that they were "protesters rather than hooligans", his official spokesman said.
Prime Minister John Key met Putin in Bali last month and said the Russian leader "gave me an assurance it [the case] would flow through the appropriate Russian judicial system" and that "in the end people need to consider their actions - the risks or otherwise - and I'm sure those protesters thought about all those issues".
Cameron told BBC Radio "they are not hooligans, they are protesters" but said nations needed "tough rules" to protect oil platforms.
He added: "Their charges have been dropped from piracy to hooliganism, but I still think that is excessive. They are not hooligans, they are protesters.
"I totally understand that countries have to have some quite tough rules to stop people invading oil platforms, but I have appealed to Vladimir Putin to try to de-escalate this and make sure that these people can go home."
His intervention came just hours before Russia said that the activists would now face the additional charge of resisting law officers - which carries a maximum sentence of five years.
Cameron's spokesman said Putin "understands" the Prime Minister's concerns and that the pair had agreed to stay in touch.
"This is now in the hands of the judicial authorities in Russia," the spokesman said. "We have made the representations that we have and we are going to use our diplomatic influence to continue ensuring that the Russian authorities understand the importance that we attach to resolving this."