Paul McCartney has appealed to Russian president Vladimir Putin to free the 'Arctic 30', who are being held in the country.

The former Beatle has written an open letter on his official website Paulmccartney.com urging Putin to free 28 activists for Greenpeace and two journalists who were arrested and charged on suspicion of piracy and hooliganism following a protest against oil drilling in the Arctic ocean earlier this year.

He wrote: "Vladimir, millions of people in dozens of countries would be hugely grateful if you were to intervene to bring about an end to this affair. I understand of course that the Russian courts and the Russian Presidency are separate. Nevertheless I wonder if you may be able to use whatever influence you have to reunite the detainees with their families?"

The group were originally held in the city of Murmansk but have recently been transported to pre-trial detention centres in St Petersburg.

This is not the first time the singer has spoken out against Russian authorities. He has been a keen supporter of the jailed Pussy Riot members. Russian authorities confirmed earlier this week that band member Nadezhda Tolokonnikova had been transferred to a penal colony in Siberia. Earlier this autumn, she suspended her nine-day hunger strike, which she began to campaign against the alleged inhumane working conditions she says she had been forced to endure at Russia's Penal Colony No 14.

Tolokonnikova and Maria Alyokhina are both serving two-year sentences for breach of public order motivated by religious hatred. The sentences were handed to them in August 2012 after the band performed their now infamous 'punk prayer' protest against President Vladimir Putin at the Cathedral of Christ The Saviour in Moscow in February 2012. They are set for release next year.