Russia criticised Britain today for failing to allow its investigators inquiring into the murder of Alexander Litvinenko to travel to London and interview witnesses.

Yury Fedotov, the Russian Ambassador, said that three weeks after relaying a request from Moscow to the Home Office for permission for Russian detectives to visit London, he had still not received an answer. “The Russian side co-operated with Scotland Yard," he said. They expect the same degree of co-operation.

“I’ve forwarded a letter from the office of the Prosecutor General (in Moscow) to the Home Office three weeks ago and for the time being no Russian investigators are in London,” Mr Fedotov said. “When British investigators asked to go to Moscow they did it Saturday morning and Monday morning they were in Moscow already.”

Part of the problem could be that the Prosecutor General’s letter setting out how to proceed with its investigation runs to 105 pages, all in Russian.

The Russian investigation also poses a delicate diplomatic problem for the British authorities. Scotland Yard detectives were allowed to travel to Moscow last year and interview witnesses in the case, including figures suspected of involvement in the poisoning of the former Russian intelligence officer with deadly polonium-210.

The Kremlin has denied any involvement in the murder. There are fears that the Russian investigators will seek to shift the focus of the inquiry away from Moscow and towards Mr Litvinenko’s contact in the Russian exiled community in London.

Boris Berezovsky, a billionaire businessman and outspoken opponent of President Putin, is expected to be high on the list of people the Russian detectives will want to question. Today he said that he had “no confidence in the integrity” of the Russian investigation, but was willing to meet Russian officers if asked to by the British authorities.

“My priority has always been to see the UK authorities be allowed to take this case to a conclusion." he told The Times. "But I would meet Russian investigators only if they met various conditions,.

“They would need to be searched for weapons and poisons. I would not meet them at the Russian Embassy. UK authorities would need to be present to ensure my protection. I would not agree to sign a confidentiality agreement about my discussion with the Russian investigators,” he said.

Akhmed Zakayev, the Chechen separatist leader, who was Litvinenko’s friend and neighbour in North London, said that he too would co-operate with the Russians if asked to by the British authorities. But he called on Scotland Yard not to let the Russian investigation undermine the hunt for Litvinenko’s real killers.

“Alexander Litvinenko was an ally in the struggle for peace and reconciliation in Chechnya. His death is a loss to our cause," he said. "In Alexander’s name, the British Government must leave no stone unturned in the search for his killer, and take whatever steps are necessary to bring them to justice.

“The extent to which Putin’s political critics are at risk in Russia and abroad constitutes a complete violation of human rights and civil liberties," he said.

The Duma, the lower house of the Russian Parliament, passed a law today making it easier for the authorities to seek the extradition of fugitives abroad.

Boris Gryzlov, the Speaker, said that the legislation, which passed 377 to 0, could make it possible to pursue Mr Zakayev through international law. The Chechen rebel leader was granted asylum in Britain in 2003.

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