Police probing the death of Russian ex-spy Alexander Litvinenko have found above-normal levels of radiation at three locations in London.
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.
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.
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".
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
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.
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.
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
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.