Senior figures in the Russian establishment lined up to implicate Mr Berezovsky, who employed and funded the former KGB spy.

The billionaire, who has been granted asylum in Britain, last night issued a statement mourning Mr Litvinenko’s death and saying that he had “complete faith” that Scotland Yard would conduct a “thorough and professional investigation”.

Detectives are understood to want to question Mr Berezovsky in further detail about the events of November 1, the day that Mr Litvinenko fell ill.

Mr Berezovsky has declined to explain publicly why Mr Litvinenko, who was recently given British citizenship, visited his headquarters in Mayfair that day.

The billionaire has accused President Putin’s regime of being behind the murder.

In his first comment on the Litvinenko affair Tony Blair yesterday insisted that no “diplomatic or political barrier” would be permitted to obstruct the police inquiry, even if the evidence pointed to a statesponsored killing.

The Prime Minister, who was on a brief stopover in Copenhagen on his way to the Nato summit in Riga, said that Mr Litvinenko’s death was being treated as a “very, very serious matter”.

He added: “We are determined to find out what happened and who is responsible.”

The Russian Prosecutor General’s Office declared that it was ready to assist the British police. A spokesman said that British detectives would be welcome to come to Moscow and would receive the Government’s full co-operation.

Mr Blair has been kept informed of developments in the inquiry, but he is not scheduled to meet President Putin in Riga. So far the Prime Minister has not spoken to Mr Putin about the case, but will do so when the time is “appropriate”.

Mr Putin has strenuously denied that the Russian authorities had anything to do with Mr Litvinenko’s death.

Police have questioned Mario Scaramella, an Italian nuclear expert who met Mr Litvinenko at a sushi bar in Piccadilly, where evidence of the radioactive poison was found. So far polonium-210 has been found at seven locations across London.

At one of those sites — 25 Grosvenor Street, the offices of Erinys, an international security company — a spokesman said that Mr Litvinenko did not work for them but had been visiting a friend there.

As Kremlin sources made their claims against Mr Berezovsky, a number of prominent politicians in Moscow named him publicly as a key figure in the affair.

Konstantin Kosachev, head of the Duma’s foreign affairs committee, said that Mr Litvinenko was linked with “certain oligarchs, including Mr Berezovsky, who in recent years have been deprived of the chance to buy corrupt power with stolen money, and apparently cannot accept this”.

In the past Russia has tried to extradite him on financial charges but the request was refused by Britain after Mr Berezovsky argued that the charge was politically motivated.

Valery Dyatlenko, a deputy head of the security committee in the Duma, Russia’s lower house, told state television: “The death of Litvinenko — for Russia, for the security services — means nothing . . . I think this is another game of some kind by Berezovsky.”

Toxicologists tested for the presence of polonium-210 at more locations in Central London that had been visited by Mr Litvinenko on the day he fell ill.

Eight people have been sent for further tests at a specialist clinic by the Health Protection Agency to check for contamination. The agency has received more than 1,100 calls from members of the public worried that they might have been exposed to radiation, but officials insisted that there was little likelihood of any risk.

As the scale of the scare grew, John Reid, the Home Secretary, issued another statement that the risk to the public was minimal.

Special precautions will be in place for a Home Office pathologist to carry out a postmortem examination as well as a “special examination” of Mr Litvinenko’s body on Friday.

Andrew Reid, the Inner North London Coroner, said that the examination was necessary to fully investigate the cause of death.

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