The Kremlin has linked its co-operation in solving the murder of Alexander Litvinenko to British help in extraditing exiled critics of President Putin.

Russian officials demanded that Boris Berezovsky, a billionaire businessman, and the Chechen separatist envoy Akhmed Zakayev should be sent back.

The issues were raised at a meeting in Moscow between Alistair Darling, the Trade and Industry Secretary, and Alexei Kudrin, the Russian Finance Minister, to discuss trade links. Mr Darling is the first Cabinet minister to visit Moscow since Litvinenko was poisoned with radioactive polonium210 in London in November.

“We did discuss the case and, in parallel, the Russian requests for their own extraditions. In relation to the Russian requests, I said very clearly that that was a matter for the British courts and ministers can’t intervene,” Mr Darling said.

“In relation to the Litvinenko case, the Crown Prosecution Service are looking at that and will decide on the evidence what action is appropriate.

“What Mr Kudrin and I did agree was very important was that both countries ought to cooperate with each other.

Russia has asked Scotland Yard if they would cooperate with Russian authorities and John Reid [the Home Secretary] has indicated that would be the case.”

Three attempts have been made by Russia to extradite Mr Berezovsky, who has political asylum in Britain and is one of the most vocal critics of Mr Putin. British courts have rejected the application each time.

Russian authorities appear determined to link the inquiry by Scotland Yard into two Russian businesmen who are suspected of involvement in Litvinenko’s death with British willingness to assist in handing over Mr Berezovsky, who is wanted for crimes in Moscow.

British detectives interviewed Andrei Lugovoy, a former KGB officer, and his business partner Dmitri Kovtun in Moscow.

They have sent a file to the Crown Prosecution Service, which is understood to have identified them as key suspects in Litvinenko’s killing.

A decision to prosecute will require the extradition of both men, which Russia says is impossible. The Russian Constitution bars the extradition of citizens to face trial abroad.

Yuri Chaika, the Prosecutor-General, has said that Scotland Yard will be allowed to return to Moscow only once his team of prosecutors has been allowed to question potential suspects in London.

The Foreign Ministry in Russia complained on Wednesday that Britain was obstructing its attempt to send prosecutors to London to interview more than 100 people, including Mr Berezovsky and Mr Zakayev.

Mr Darling said that the British Government was willing to grant permission for the Russian team to visit.

Litvinenko accused Mr Putin from his deathbed of ordering his assassination, a charge that is denied strongly by the Kremlin. It accuses Mr Berezovsky of carrying out the killing.

Mr Darling was leading a delegation of chief executives from 20 British companies and Richard Lambert, the Director-General of the CBI, for talks on investment in Russia’s booming economy.

Business relations between the two countries are strained amid concerns about Kremlin tactics in exerting pressure on Shell to concede a majority stake in the Sakhalin-2 oil and gas project to Gazprom in December.

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