Britain's intelligence agencies last night claimed that the poisoning of the Russian dissident Alexander Litvinenko bore the hallmarks of a "state-sponsored" assassination.

A senior Whitehall official told The Times that confirmation that the former Russian spy, who had become a British citizen, had been poisoned with radioactive polonium-210 and other evidence so far not released pointed to the murder being carried out by foreign agents.

Last night the Foreign Office said that officials had met with the Russian ambassador in London and had asked the Kremlin to hand over any information that it had which could help the Scotland Yard investigation.

Cobra, the Cabinet's emergency security committee, met yesterday after toxicologists confirmed that the 43-year-old former KGB colonel had a large dose of alpha radiation in his body. The committee chaired by John Reid, the Home Secretary, considered the risk to the public after the discovery of radioactive material in a Central London sushi bar and at the Millennium Hotel, near the US embassy in Grosvenor Square, where Mr Litvinenko held meetings on November 1. Radioactive traces were also found at his family home in Muswell Hill, North London.

The quantity of polonium-210 used could only have been obtained from a nuclear instillation, scientific experts said.

A senior Whitehall official said: "Cobra met because thousands of people have passed through the sushi bar in the past three weeks and there is a potential risk for the public and we have to examine all the implications."

Experts from the Government's Health Protection Agency tried to allay public fears by stressing that it was unlikely that friends, family and medics who were with Mr Litvinenko at University College Hospital had been contanimated.

Security sources said that MI5 and MI6 were engaged in a "joint enterprise" with Scotland Yard in what was "an unprecedented death" in Britain. Anti-terror squad Continuedetectives refused to say where the deadly element was placed, or in what quantities they found it at the Itsu sushi bar in Piccadilly or the Pine Bar of the Millennium Hotel, where the dissident met two Russians on November 1.

Mr Litvinenko's father, Walter, openly accused the Kremlin of murdering his son. They also released a statement that Mr Litvinenko dictated 48 hours before he died, blaming President Putin for his death.

Mr Litvinenko told the Russian President: "You may succeed in silencing one man, but the howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life.

"May God forgive you for what you have done, not only to me, but to beloved Russia and its people."

Mr Putin interrupted preparations for an EU/Russia summit in Helsinki to deny involvement. He criticised Mr Litvinenko's entourage, the media, the British secret service and even the Italian Mafia. He claimed that the letter accusing him of being "barbaric and ruthless" was a forgery concocted by Mr Litvinenko's wife and father: "If this note was produced before the death of Mr Litvinenko, I wonder why it was not published when he was alive?"

Mr Litvinenko's funeral will be held in London.