The offices of an exiled billionaire were sealed off tonight in London as police investigating the death of former Russian spy Alexander Litvinenko found traces of a deadly radioactive substance.
Evidence of polonium 210 were found in the London office of Mr Litvinenko friend, the tycoon Boris Berezovsky.
Police also sealed off the headquarters of an international security company visited by Mr Litvinenko after finding traces of the substance.
Alexander Litvinenko, former KGB spy and author of the book
Scotland Yard said traces of the substance were found at 7 Down Street and 25 Grosvenor Street, as they continued to probe Mr Litvinenko's death from radiation poisoning on Thursday evening.
A "significant quantity" of polonium 210 was found in Mr Litvinenko's urine following extensive tests by forensic toxicologists.
The new finds come after traces of the substance were also detected at the Itsu sushi restaurant in Piccadilly, the Millennium Hotel in Grosvenor Square - both visited by Mr Litvinenko - as well as his home in Muswell Hill, north London.
Three people have also been sent for radiological tests as a precautionary measure following the radiation poisoning of Mr Litvinenko, Home Secretary John Reid said today.
Mr Litvinenko, a fierce critic of Russian President Vladimir Putin, died from radiation poisoning last Thursday night.
Scotland Yard said only that traces of polonium 210 had been found at offices at 7 Down Street and 25 Grosvenor Street but Mr Litvinenko's friend Alex Goldfarb confirmed tonight that the address in Down Street was the office of Mr Berezovsky.
Health officials have said Polonium 210 is dangerous only if it is swallowed, inhaled or absorbed through a wound. Mr Reid said there was "no need for public alarm".
But his comments deepened the mystery over how and where Mr Litvinenko ingested the poison that caused his agonising death, causing his hair to fall out and gradually shutting down his vital organs.
Some 500 people have called a hotline for health advice since the death, Mr Reid said.
A spokeswoman for Britain's Health Protection Agency said three people had been referred to a clinic for radiological assessment. She declined to comment on their symptoms.
Police are investigating the case as a suspicious death and the affair has raised tension between London and Moscow. The Kremlin has described Mr Litvinenko's murder allegation as ridiculous, and Mr Putin said the death was being used for "political provocation".
Northern Ireland Secretary Peter Hain yesterday condemned "murky murders" that had taken place in Mr Putin's Russia and criticised "huge attacks" there on individual freedoms and democracy.
But Downing Street said today it was premature to jump to conclusions.
Mr Litvinenko left the KGB's main successor, the Federal Security Service, and came to Britain with his wife and son in 2000; he was granted asylum and became a British citizen last month.
Local authorities said an inquest into his death was expected to open on Thursday.