Detectives investigating the poisoning of Alexander Litvinenko found traces of a radioactive poison at the offices of the billionaire and fellow exile Boris Berezovsky last night.

Police sealed off the Mayfair office after finding evidence of polonium 210, a “significant quantity” of which was found in Mr Litvinenko’s urine.

The discovery at 7 Down Street forms part of a radioactive trail left around London by Mr Litvinenko, 43, as police try to discover where the former KGB spy was poisoned.

They are using his mobile telephone records and closed-circuit television cameras to plot his route as he met wealthy Russians, business partners and nuclear experts on the day that he fell ill.

It appears that he left a trace of the radioactive isotope nearly everywhere he visited. It has been found at seven locations, including the two hospitals where he was treated.

Mr Litvinenko was able to tell police most of the places he stopped at and who he met. Officers are checking surveillance cameras to see whether anyone was following the Russian dissident, who was recently given British citizenship.

As well as the discovery in Down Street, traces of polonium 210 were found at 25 Grosvenor Street, home to Erinys, a security and risk management company that Mr Litvinenko had visited.

Mr Berezovksy, a fierce critic of President Putin of Russia, has accused the Kremlin of being behind what he calls an “assassination”. He was known to be close to Mr Litvinenko and visited him in hospital before his death.

Polonium 210 was also found at a mansion block close to the Millennium Hotel that was converted into offices where Mr Litvinenko reportedly discussed a proposed business deal in oil and gas exploration with three Russians on November 1. Part of the intensive care ward at University College Hospital remains sealed off, as do parts of the Millennium Hotel and the Itsu sushi bar in Piccadilly. Three people who ate there were being tested last night for radiation poisoning.

Health and safety experts said that the traces of polonium 210 found so far pose a minimal threat to the public.

A senior official at the Health Protection Agency said that it hoped that computer models would be able to measure the strength and amount of the toxin to deduce when Mr Litvinenko was poisoned.

More than 500 people have contacted a health hotline for advice. Three showed symptoms that could be related to radiation sickness.

A shop in Piccadilly where Mr Litvinenko bought a newspaper was also searched yesterday and no traces of the radioactive material were found.

John Reid, the Home Secretary, said in a statement to the Commons that Russia had been asked to co-operate.

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