Traces of polonium-210 radiation have been found at two more central London addresses, police probing ex-Russian spy Alexander Litvinenko's death say.
Mr Litvinenko was a fierce critic of Russian President Vladimir Putin
One address, in Down Street, reportedly houses the offices of his friend, exiled billionaire Boris Berezovsky.
The other location, in Grosvenor Street, is the headquarters of security and risk management company Erinys.
Traces of the substance have already been found at a sushi restaurant, hotel and Mr Litvinenko's north London home.
Three people who have either been to the venues or had contact with him are to undergo radiological tests.
Mr Litvinenko's friend Alex Goldfarb said the address in Down Street were the offices of Mr Berezovsky.
He said: "I have been to that office many times, Mr Litvinenko did and everyone who was friends with Mr Berezovsky because that's his principal place of business in London."
Erinys said that in the light of recent events the company had "immediately contacted" the police to tell them of a visit made by Mr Litvinenko to its offices.
TRACES OF RADIATION
The places where polonium-210 radiation has already been found are the Itsu sushi restaurant in Piccadilly, the Millennium Hotel in Grosvenor Square and Mr Litvinenko's home in Muswell Hill.
The death of the 43-year-old former KGB colonel last Thursday has been linked to the discovery of polonium-210 in his body.
Home Secretary John Reid told MPs that Russia had been asked to co-operate in the inquiry into Mr Litvinenko's death.
In an emergency statement in the Commons on Monday, Mr Reid said the Russian ambassador had been called to the Foreign Office at the end of last week.
"He was asked to convey to the Russian authorities our expectation that they should be ready to offer all necessary co-operation to the investigation as it proceeds," said Mr Reid.
Mr Reid also chaired Monday's meeting of the special emergency "Cobra" committee, which brings together ministers, officials and experts, to assess the risk to the public.
The Health Protection Agency said more than 450 people had called a government hotline for advice and 18 had been followed up.
Three have been referred to a specialist clinic as a precautionary measure because they had symptoms which may indicate radiation poisoning.
It is thought they contacted the NHS helpline and answered detailed questions about their condition before being referred for a face-to-face consultation and possible urine test.
Results are expected later in the week.
An inquest into Mr Litvinenko's death will be opened and adjourned on Thursday at St Pancras Coroner's Court, said a Camden Council spokesman.
Mr Litvinenko, 43, became a British citizen after coming to live in the UK.
Friends have suggested Russian top-level involvement in his death because Mr Litvinenko was a critic of Russian President Vladimir Putin.
Mr Litvinenko visited Itsu on 1 November
And on Sunday Northern Ireland Secretary Peter Hain said "murky murders" had cast a shadow over Mr Putin's achievements.
But the Kremlin has repeatedly dismissed allegations of involvement in the death as "sheer nonsense".
Asked about Mr Hain's comments, Tony Blair's official spokesman said the prime minister had made clear his concerns about some aspects of human rights in Russia but it would be premature to draw conclusions in this case.
Mr Litvinenko had been investigating the murder of prominent Russian journalist Anna Politkovskaya, another critic of the Putin government, before he fell ill.
On the day he was taken ill, he had had meetings at the restaurant and the hotel's Pine Bar.