Three more people have tested positive for the radioactive substance thought to have killed former Russian spy Alexander Litvinenko.
Alexander Litvinenko died in a London hospital last month
Tests on two members of staff at the Millennium Hotel and one at the Sheraton Hotel, both in London, showed low-level exposure to polonium-210.
Ten people in the UK have shown traces of the substance since Mr Litvinenko died in a London hospital last month.
Friends believe he was deliberately poisoned with polonium-210.
Previously, Mr Litvinenko's wife Marina and seven members of staff at the Millennium Hotel's Pine Bar tested positive for the material, although one of the hotel workers subsequently showed normal levels of polonium radiation.
The Health Protection Agency (HPA) said of the latest positive tests: "These cases are related to areas which have been sealed off to the public as part of the police investigation.
"The levels are not significant enough to result in any illness in the short term and any increased risk in the long term is likely to be very small."
The three latest cases involved lower levels of radiation than that of Mrs Litvinenko, who herself had very low-level exposure.
A HPA spokeswoman added that there was "no significant public health risk" at the Millennium Hotel in Mayfair and the Sheraton Hotel in Park Lane.
The HPA said that up to Monday, 3,806 people had called helpline NHS Direct about possible exposure to polonium, with 649 cases needing more investigation, 29 referred to specialists and 21 people given the all-clear.
Mr Litvinenko, 43, met three Russian men at the Millennium Hotel on 1 November, the day he fell ill.
Investigations have also found traces of polonium-210 radiation in the Russian capital Moscow and Hamburg in Germany.
Scotland Yard detectives travelled to Moscow two weeks ago for the questioning of witnesses, and may go to Germany as well.
Russian authorities say British officers have now completed their inquiries in the country.
On his deathbed at University College Hospital, Mr Litvinenko accused Russian President Vladimir Putin of being behind his poisoning. Moscow has denied any involvement.