The HPA chief executive, Pat Troop, told a news conference the agency was dealing with "an unprecedented event in the UK": the deliberate poisoning of someone with a radioactive substance.

Mr Litvinenko, a Russian dissident who fled to the UK in 2000, died last night, three weeks after becoming ill with suspected poisoning. In a statement dictated before his death and made public today, he accused the Russian president, Vladimir Putin, of being behind his killing.

It emerged today that traces of radioactive material had been found at three London locations, including a restaurant and bar where the former KGB agent had met contacts shortly before falling ill.

Scotland Yard said police had called in expert help to search for radioactive material at the Itsu sushi restaurant and the Millennium hotel in central London, and at Mr Litvinenko's home in north London.

Earlier, it said its counterterrorism unit was investigating the "unexplained death".

The home secretary, John Reid, chaired a meeting of Cobra, the government's emergency committee, to discuss Mr Litvinenko's death, a Cabinet Office spokeswoman said.

There were unconfirmed reports that the Foreign Office had spoken to the Russian authorities today about Mr Litvinenko's death and called it "a serious matter".

HPA teams were at the two London hospitals where Mr Litvinenko was treated to assess whether staff or family who had come into contact with him were now at risk, Professor Troop said.

The postmortem examination on Mr Litvinenko's body was delayed while experts sought to establish whether it was safe to perform the procedure.

"Our role is to assess the risk to the public and those who might have been in contact with this man," Prof Troop said.

Roger Cox, the director of the HPA's centre for radiation, chemical and environmental hazards, said small doses of polonium 210 could lead to increased cancer risks later in life, but only if they were ingested by people who came into contact with a contaminated substance. At high doses there would be enough damage to bone marrow, intestines and other organs to cause them to malfunction.

He said those people who had touched Mr Litvinenko as he lay dying could theoretically have ingested the radiation but stressed that hospital procedures and normal hygiene measures would have reduced the possibility to an "insignificant level".

Mr Cox said polonium 210 occurred naturally and was present in some foods in low doses. It was a "pure alpha emitter", which meant its short radiation particles would not pass through solid substances, such as skin, but could be ingested.

Earlier, Mr Litvinenko's friend Alexander Goldfarb read out a statement dictated by the 43-year-old in the presence of his wife, Marina, on Tuesday, two days before he died.

"You may succeed in silencing me, but that silence comes at a price," the statement said. "You have shown yourself to be as barbaric and ruthless as your most hostile critics claim.

"You may succeed in silencing one man. But a 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."

Mr Litvinenko, who has been living in London since defecting from Russia six years ago, said: "I am honoured to be a British citizen."

Mr Litvinenko died in hospital just after 9pm yesterday. His condition had deteriorated rapidly after he suffered a heart attack and lost consciousness on Wednesday night.

Speaking outside the hospital this morning, his father, Walter Litvinenko, who was at his son's bedside when he died, described him as "very courageous".

"A terrible thing has happened here today," he said in Russian, wiping tears from his eyes. "He was a very honest and good man and we loved him very much. And now he is not with us.

"He was very courageous when he met his death and I am proud of my son. It was an excruciating death ... but he never lost his human dignity.

"Marina and [Alexander] were the most wonderful couple. They were so happy here in London, but the long hand of Moscow got them here on this soil. This regime is a mortal danger to the world."

Since Mr Litvinenko became ill, his friends and family have insisted that he was the victim of a Moscow plot.

The former KGB agent was a fierce critic of Mr Putin's government and was investigating the murder of his friend, the Russian journalist Anna Politkovskaya, when he became ill.

But both the Kremlin and Russia's foreign intelligence service, the SVR, have denied any involvement.

President Putin hit back this afternoon at claims he was behind the death, saying there was no proof that it was "violent".

Speaking, at an EU-Russia summit in Helsinki for the first time since Mr Litvinenko's death, he said: "There's no grounds for speculation of this kind." He sent his condolences to the former spy's family.

He added: "I hope the British authorities would not contribute to instigating political scandal. it has nothing to do with reality."

If Russia is found to have had a hand in the poisoning, there could be serious diplomatic consequences. It would be the first such incident known in the west since the end of the cold war.

Mr Litvinenko's friend Oleg Gordievsky, another former spy who defected to Britain, said his friend was a victim of "revenge and malice" of "evil forces" in Russia.

One doctor said the type of poison used may never be known. Earlier suggestions that it was a heavy metal such as thallium or a radioactive substance were dismissed yesterday.

A Russian intelligence officer said today that he was ready to answer questions from British police over a meeting he had with Mr Litvinenko the day before he fell ill.

The man, a former officer in the FSB state security service, told a Russian newspaper that he met Mr Litvinenko in the bar of a London hotel on November 1 along with two other Russian acquaintances.

The next day, Mr Litvinenko called him to cancel another meeting because he felt unwell, he told Kommersant.

Mr Litvinenko fled to Britain in 2000 with his wife and son after, he claimed, he refused to carry out an order from senior FSB officers to murder the Russian billionaire Boris Berezovsky.

Four years ago, he co-wrote a book that alleged that FSB agents were behind the bombing of blocks of flats in Russia that killed more than 300 people in 1999 and were blamed by Russian officials on Chechen guerrillas.