Mr Litvinenko's condition deteriorated rapidly in hospital
Radiation briefing
Mr Litvinenko's death has been linked to the presence of a "major dose" of radioactive polonium-210 in his body.
Scotland Yard confirmed traces were also found at his home, a sushi bar and a hotel, but the risk to others was said by health experts to be very low.
The Kremlin has denied UK citizen Mr Litvinenko's claims it was involved.
The traces were found at the Itsu sushi restaurant in Piccadilly, the Millennium Hotel, Grosvenor Square, and at Mr Litvinenko's home in Muswell Hill, north London, Scotland Yard said.
Information
Moscow has been asked to help British police in their investigations, the Foreign Office has said.
Officials discussed the issue with the Russian Ambassador, Yuri Fedotov, at a meeting this afternoon, said a spokeswoman.
Officers are looking at CCTV footage and interviewing witnesses, trying to find out who he met around the time he fell ill on 1 November, said Peter Clarke, head of the Counter Terrorism Command which is leading the investigation.
Tests are also being carried out at the two London hospitals where Mr Litvinenko had been treated, University College and the Barnet General, the Health Protection Agency said.
Professor Pat Troop from the HPA told a news conference that the tens of hospital staff who had come into contact with him would be monitored.
RADIATION TYPES Alpha particles are stopped by a sheet of paper and cannot pass through unbroken skin Beta particles are stopped by an aluminium sheet Gamma rays are stopped by thick lead
She said Mr Litvinenko would have had to either eaten, inhaled or been given the dose of polonium-210 through a wound.
She said the nature of death as an "unprecedented event in the UK".
Roger Cox from the HPA said a large quantity of alpha radiation emitted from polonium-210 had been detected in Mr Litvinenko's urine.
The radiation cannot pass through the skin, and must be ingested or inhaled into the body to cause damage.
He said people who came into contact with Mr Litvinenko's excreta - including sweat - could in theory be affected, but described the risk as "insignificant".
POLONIUM-210 a highly radioactive and toxic element present in foods and tobacco in low doses small amounts occurs naturally in the body can be manufactured using the bombardment of neutrons has industrial uses such as in anti-static devices very dangerous if significant dose ingested external exposure not a risk
'No radiation risk' to public What is polonium-210?
As the conference drew to a close, a heckler interrupted saying he was from Ukraine and had also been the victim of poisoning.
A post-mortem examination on Mr Litvinenko has not been held yet.
The delay is believed to be over concerns about the health implications for those present at the examination.
The Home Office said anybody concerned should contact NHS Direct on 0845 4647, who have been briefed about the issues.
Meanwhile, the government's civil contingencies committee Cobra has met to discuss the case.
'Sheer nonsense'
Friends have said Mr Litvinenko was poisoned because of his criticism of Russia.
In a statement dictated before he died at University College Hospital on Thursday, the 43-year-old accused Russian President Vladimir Putin of involvement in his death.
Mr Litvinenko had recently been investigating the murder of his friend, Russian journalist Anna Politkovskaya, another critic of the Putin government.
LITVINENKO TIMELINE 1 Nov - Alexander Litvinenko meets two Russian men at a London hotel and then meets Italian academic Mario Scaramella at a sushi bar in Piccadilly. Hours later he falls ill and is admitted to Barnet General Hospital 17 Nov - Mr Litvinenko is transferred to UCH 19 Nov - Reports say Mr Litvinenko is poisoned with thallium 21 Nov - A toxicologist says he may have been poisoned with "radioactive thallium" 22 Nov - Mr Litvinenko's condition deteriorates overnight. Thallium and radiation ruled out 23 Nov - The ex-spy dies in intensive care
Litvinenko statement in full Reaction: Russian's death Timeline of case in full
Mr Putin's spokesman Dmitry Peskov reiterated the Kremlin's earlier dismissal of allegations of involvement in the poisoning as "sheer nonsense".
Mr Putin himself has said Mr Litvinenko's death was a tragedy, but he saw no "definitive proof" it was a "violent death".
Police have been examining two meetings Mr Litvinenko had on 1 November - one at a London hotel with a former KGB agent and another man, and a rendezvous with Italian security consultant Mario Scaramella, at the sushi restaurant in the West End.
Mr Litvinenko, who was granted asylum in the UK in 2000 after complaining of persecution in Russia, fell ill later that day.
In an interview with Friday's Telegraph newspaper, former KGB bodyguard Andrei Lugovoi said he had met Mr Litvinenko at the Millennium Hotel but vigorously denied any involvement in the poisoning.
Mr Scaramella, who is involved in an Italian parliamentary inquiry into Russian secret service activity, said they met because he wanted to discuss an e-mail he had received.
Mr Alexander Litvinenko, reputed to have been an Ex-Russian spy who had defected to Britain, died last night in mysterious circumstances. He had alleged many associations between people in high places and organised international crime, implying that President Putin and Romano Prodi among others had been involved personally. He had sought political asylum in UK in 2000 and became a naturalised citizen in October this year, just weeks before his death.

Mr Litvinenko was said to have been investigating the shooting of Russian journalist and human rights activist Anna Politkovskaya, a well-known critic of Russian activities in , in her apartment in Moscow October 7, 2006. It is reported that he had met two Russians in a hotel room, one said to be former member of the  in connection with the affair and he went on to meet Mario Scaramella at a  bar in Piccadilly where some papers were exchanged. Some hours after this he was taken ill. 

Mr Litvinenko was admitted to , north London on November 1, 2006 complaining of feeling sick. By November 11, he was said to be suffering from ''serious poisoning''. A week later he was transferred to  in central London. A week later he was said to have been poisoned by ingesting , once used in rat poison, but, in the opinion of some doctors, there were signs of , including loss of weight and shedding of hair. Various explanations of his condition were offered. Last night he suffered a heart attack, after having left a message blaming President Putin for having him killed. 

The radioactive  -210 was found in his blood and urine as reported by the . The post-mortem was cancelled. Subsequently the Agency examined conditions in the hospitals in which Mr Litvinenko has been treated. Police visited the Itsu sushi restaurant in Piccadilly, his home in Muswell Hill and the Millennium Hotel, Grosvenor Square where the meeting on November 1 had been held. Traces of radiocactivity were found in all three places. It is speculated that the polonium was probably eaten by Litvinenko as a substance that could be combined with a salt-like substance, such as .

It was reported that a meeting had been held in Cabinet Office Briefing Room A (COBRA) used for high level emergency planning and control, to consider the implications of these events. The Foreign Office asked Moscow for a response to the accusation of Russian involvement and President Putin himself dismissed the allegations saying (before the cause of death had been established) that there was no proof of an unnatural death and that the case was being used as a "political provocation".