British Prime Minister Tony Blair promised on Tuesday that "no diplomatic or political barrier" would be allowed to hamper an investigation into the death of a former Russian spy killed by radiation poisoning.
He spoke as police said they were searching two more central London addresses in connection with the death of Alexander Litvinenko, after finding traces of radiation at several sites where the former agent-turned-Kremlin critic had been.
Mr Blair said the case was "very serious" and he would discuss it with Russian President Vladimir Putin in person if necessary.
"I haven't spoken to President Putin but I will do so at any time that is appropriate," Mr Blair told a news conference in Copenhagen.
"There is no diplomatic or political barrier in the way of [the] investigation going wherever it needs to go. It obviously is a very serious matter and we are determined to find out what happened and who is responsible."
Significant amounts of radioactive Polonium 210 were found in the body of Mr Litvinenko. He died on Thursday, accusing Mr Putin of ordering his slow, agonising death. He had received British citizenship just weeks earlier.
Police are treating his death as suspicious.
The Kremlin denies any involvement but the Litvinenko case has fuelled tensions between London and Moscow.
A post mortem will be carried out on Friday under secure conditions to avoid possible radioactive contamination.
Mario Scaramella, an Italian KGB expert who met Mr Litvinenko the day he became ill, is under British police protection and is undergoing medical tests.
Radiation was found at the sushi restaurant where Mr Litvinenko met Mr Scaramella on November 1st and traces have also been detected at several more sites, including Mr Litvinenko's home, a hotel he visited, the offices of Russian tycoon Boris Berezovsky and the offices of Erinys, a security and risk management company.
London police said they were also searching the Sheraton Park Lane Hotel Piccadilly and another London address.
Mr Berezovsky said he had complete faith in police and said he was "deeply saddened" at his friend's death. Eight people have been sent for precautionary radiological tests, Britain's Health Protection Agency said.
Russia's nuclear chief said he doubted the Polonium 210 which apparently killed Mr Litvinenko had been stolen from Russia.
"The Polonium that is produced in Russia for export ... is controlled very strictly, there is one producer and so it is under very strict control in accordance with all international agreements," Sergei Kiriyenko said at a briefing.
There is frenzied speculation in British and Russian media about possible motives for killing Mr Litvinenko. Some link it to his reported probes into Russian oil company Yukos and into last month's murder of Russian journalist Anna Politkovskaya.