The Foreign Office last night rebuffed the sensational claim by Andrei Lugovoi, a former Russian spy accused of poisoning defector and ex-KGB agent Alexander Litvinenko, that MI6 was involved in the murder.
It insisted the matter was a criminal one and "not an issue about intelligence".
A spokesman said: "A British citizen was killed in London and UK citizens and visitors were put at risk. Our position is clear. A request for the extradition of Mr Lugovoi to face trial in a UK court has been handed over. We await the formal Russian response."
However, the chances of Mr Lugovoi facing a British judge and jury appear to be nil as Moscow has made clear it has no intention of handing him over because it would breach the Russian constitution for a national to face trial abroad.
His assertions, though, are likely to lead to a further chilling of relations between London and Moscow. Konstantin Kosachev, a senior Kremlin-connected Russian MP, responded by urging the UK authorities to help investigate the "very serious accusations against British secret services".
At a press conference in the Russian capital, Mr Lugovoi claimed that he had evidence the British security services had a hand in the murder of Mr Litvinenko, who died last November after being poisoned with Polonium 210 in London.
In yet more claims reminiscent of a complicated Cold War spy thriller, the former Russian agent also alleged British intelligence tried to recruit him to get "compromising information" about President Putin and that both Mr Litvinenko and Boris Berezovsky, the Russian tycoon now living in London, were on its payroll.
However, the allegations were immediately dismissed by Mr Berezovsky, who described them as further proof Mr Lugovoi was acting under Kremlin instructions while Alex Goldfarb, a close friend of Mr Litvinenko, insisted Mr Lugovoi's statement illustrated once again how the Russian government was behind the defector's death.
In Moscow, Mr Lugovoi told reporters: "It's hard to get rid of the thought that Litvinenko was an agent who got out of the (British) secret service's control and was eliminated. Even if it was not done by the secret service itself, it was done under its control or connivance."
Later, he added that British spies had "asked me to collect compromising information on President Putin".
When asked if he could back up the claims, Mr Lugovoi said: "I have evidence" yet did not elaborate.
In response, Mr Berezovsky said it was "now clearer than ever that the Kremlin is behind the murder of Alexander Litvinenko". He went on: "Everything about Mr Lugovoi's words and presentation made it obvious he is acting on Kremlin instructions."
On Monday, papers calling for Mr Lugovoi's extradition were delivered to Russia after the Crown Prosecution Service's announcement that, following the police investigation, Mr Lugovoi was being charged with Mr Litvinenko's murder.
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