Traces of the extremely toxic and radioactive element were also found at Alexander Litvinenko's home in north London, Scotland Yard said.

 Litvinenko, 43, met two Russian acquaintances at the hotel before going on to the sushi bar on November 1. He then fell ill and was treated at two hospitals before he died late Thursday.

 The chief executive of the Health Protection Agency Professor Pat Troop told Sky News television earlier that only a "small amount" of radiation was found at the sushi bar and hotel, without naming the substance.

 The HPA announced at a news conference earlier Friday that a "large quantity" of alpha radiation "probably from a substance called polonium 210" had been detected in Litvinenko's urine.

 Police said in a statement that the material was identified following "extensive tests" by forensic toxicologists.

 "We continue to carry out a thorough investigation into the circumstances surrounding Mr Litvinenko's death." said Deputy Assistant Commissioner Peter Clarke, head of the force's counter-terrorism unit who is leading the probe.

 "Detectives are carrying out an intensive investigation. We will trace possible witnesses, examine Mr Litvinenko's movements at relevant times, including when he first became ill and identify people he may have met.

 "There will also be an extensive examination of CCTV footage."

 No arrests have been made so far, he added.

 Troop told Sky News the risk to the general public was minimal and they were concentrating efforts on the hospitals where Litvinenko was treated.

 Clarke said they were working closely with the HPA to reassure the public and to ensure public safety.

Agence France Presse