Officers said that Mr Litvinenko, a strong critic of President Putin, had been placed under police guard as he continued fighting for life in a London hospital.

The effects of the poison were graphically illustrated by pictures of the ravaged physical appearance of the former agent of the FSB, the Russian state security organisation, as he sat in his hospital bed.

The announcement yesterday of the involvement of Scotland Yard’s Counter-Terrorism Command underlined the seriousness with which the attempted murder of Mr Litvinenko, 41, was being treated, and the possible diplomatic implications if a Kremlin connection were uncovered.

In Moscow a spokesman for the SVR, the Russian foreign intelligence service, said: “We have nothing to do with what happened to Litvinenko. The Russian secret services have not in a long time carried out poisonings or any form of assassination.”

However, the poisoning of Mr Litvinenko with thallium, a soft, malleable metal, has now been raised by the Foreign and Commonwealth Office in discussions with the Russian Embassy in Kensington, London.

The Foreign Office and Britain’s security and intelligence services, MI5 and MI6, are waiting to see what the Metropolitan Police uncover before taking any action. Initially, a detective superintendent from the Specialist Crime Directorate was put in charge of the inquiries and had begun seeking two Russians who met Mr Litvinenko on the day that he became ill. One of them was a former KGB officer well known to Mr Litvinenko; the other was a stranger to him.

The police, who have both their names, also appointed a liaison officer to keep in touch with Mr Litvinenko’s wife, Marina, and their son, Anatoli.

A Scotland Yard spokesman said that the decision to involve the Counter-Terrorism Command, led by Deputy Assistant Commissioner Peter Clarke, was the “appropriate course of action”.

The Specialist Crime Directorate remains in support.

Mr Litvinenko, who was a lieutenant-colonel in the FSB, the successor to the KGB, fled Moscow in 2000. Yesterday he was moved to the intensive care unit at University College Hospital after his health deteriorated overnight and is under police guard.

He was still able to be interviewed by police. Scotland Yard confirmed that his illness was being treated as a deliberate poisoning and that detectives were awaiting the results of toxicology tests.

The Foreign Office official was careful to say that the Russian Embassy had been spoken to only routinely and that the media interest in the poisoning had been raised as “a matter of observation” with the Russians.

However, in Moscow, Dmitri Peskov, a senior Kremlin spokesman, described the accusations of official Russian involvement as “sheer nonsense”.

The poisoning is alleged to have taken place on November 1, but Scotland Yard confirmed that it became involved only on November 17. Alex Goldfarb, a friend of Mr Litvinenko, visited him in hospital yesterday. He said: “He has to be constantly monitored. His speech is a little bit distorted but he can speak without pain.”

Akhmed Zakayev, a former Chechen political leader who lives next door to Mr Litvinenko in North London, also visited him in hospital and, according to Oleg Gordievsky, the KGB officer and double agent who defected to Britain in 1985, was appalled at how he looked.

Mr Zakayev last night accused Moscow of being behind the murder attempt. He said: “Kremlin assassination tactics to remove and discredit Putin critics can no longer be ignored by the West.”

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