Russia has written to Britain officially refusing to extradite the chief suspect in the murder of a Russian emigre in London last year, Interfax news agency reported today.

The Russian Prosecutor General has ruled that Russia is legally unable to send Andrei Lugovoy, a former KGB officer turned businessman, back to Britain to face charges, said Interfax. So far there has been no official confirmation from Moscow that the letter has been sent.

“According to the country’s constitution, it is forbidden to extradite Russian citizens to foreign states,” said Interfax, quoting what it said was an informed source.

The British Crown Prosecution Service accused Mr Lugovoy in May of poisoning Alexander Litvinenko, also a former KGB officer, with polonium-210 in a London restaurant. Litvinenko died in hospital last November, three weeks after the radioactive material was administered.

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The Prime Minister will have to decide whether to take action against Russia for its failure to co-operate in the murder inquiry, or quietly to drop the matter

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The investigation into his killing was led by Scotland Yard's anti-terror squad.

The case has severely strained relations between Britain and Russia, in part because Litvinenko had received political asylum and citizenship here.

Vladimir Putin, the Russian President, has said publicly that extraditing Russian citizens would be a violation of the Constitution, and Russian officials have previously told reporters that they would not extradite Mr Lugovoy.

Yesterday Anthony Brenton, the British Ambassador to Moscow, said that five weeks after it made its request Britain was still awaiting an official response.

Today a spokesperson for the British Embassy in Moscow said: “I am not yet aware of any official response to the extradition request.”

Russia has tried to turn the tables on Britain by opening its own investigation into claims by Mr Lugovoy that the British secret services and Boris Berezovsky, a self-exiled billionaire tycoon, could have had a hand in Litvinenko’s death.

Mr Lugovoy also alleged at a press conference that MI6 had tried to recruit him to find compromising material against Mr Putin.

Last week, Russia’s FSB secret service said that another Russian national had turned himself over to Russian authorities after Mr Lugovoy’s press conference, saying that he had spied for Britain and now feared for his life.

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