Police investigating the death of the Russian agent Alexander Litvinenko found traces of the radioactive metal polonium-210 in two buildings in central London as efforts continued to trace the source of the poison.
Forensic scientists spent eight hours searching the premises yesterday, which include a four-storey Georgian town house owned by the Russian oligarch Boris Berezovsky. It is rented to companies that include a specialist security agency run by a former member of British special forces.
The address, at 25 Grosvenor Street, Mayfair, remained sealed off last night after Scotland Yard confirmed that minute traces of polonium-210, which poisoned Mr Litvinenko, were found on the fourth floor of the building. The other address, 7 Down Street, off Piccadilly, is also in Mayfair.
It is understood the buildings were singled out by investigators from the Yard's counter-terrorist branch as part of efforts to trace Litvinenko's movements on 1 November, the day he fell ill.
Mr Berezovsky, one of the first billionaires to emerge in Russia after the fall of Communism, owns 25 Grosvenor Street as part of his extensive property portfolio. He was a close ally and patron of Litvinenko, who broke rank with Russia's FSB security service to claim he had been ordered to assassinate the oligarch.
The offices at 25 Grosvenor Square, close to the Millennium Hotel, where Mr Litvinenko met three Russians, including a former KGB bodyguard on 1 November, have two security companies as tenants. One of them, Titon International, describes itself as offering a "wide range of bespoke security and intelligence services" to business clients in Britain and abroad.
The company website states that its chief executive is a former senior member of British special forces and a counter-terrorism expert. A spokesman for Titon declined to comment on whether its premises were where the polonium-210 had been found. There is no suggestion Litvinenko was poisoned in the building.
The development came as the Health Protection Agency (HPA) confirmed that three people have been sent for "very precautionary" tests at a specialist clinic to ascertain whether they have been also exposed to the radioactive substance.
Litvinenko died on Thursday from the effects of ingesting polonium-201 in what he believed was an assassination sanctioned by President Vladimir Putin of Russia. He had held meetings at the Millennium and a branch of the Itsu restaurant chain in Piccadilly on 1 November. More than 500 people have called an NHS Direct help line since doctors asked for anyone who visited the hotel and restaurant on that day to come forward.
Dr Pat Troop, the chief executive of the HPA, said: "The very small number we have so far referred to the clinic is on a very precautionary basis. We are being very, very careful."
It is understood that none of those being tested are customers or staff at Itsu, which was being decontaminated yesterday. Tests on Litvinenko's widow, Marina, are also understood to have proved negative.
It was unclear if medical staff were among those being tested. Part of the intensive care ward at University College London Hospital, where Litvinenko was treated before his death, also remained closed.