Alexander Litvinenko died on Thursday night after a three week illness that saw his hair fall out, his body waste away and his organs slowly fail. In a statement read out after his death, he accused Putin of what would be the Kremlin's first political assassination in the West since the Cold War.
''You may succeed in silencing one man. But a howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life,'' he said.
Putin shrugged off the charge as a ''political provocation'', but tremors could be felt of what could become a diplomatic earthquake if evidence supports a Kremlin link to the killing.
Britain summoned the Russian ambassador in London to ask for help solving the case, and British officials said they had found the rare radioactive substance polonium 210 in Litvinenko's body.
''What we have is an unprecedented event in the UK -- that someone has apparently been deliberately been poisoned with a type of radiation,'' Health Protection Agency chief Pat Troop told reporters.
Police said they found the substance at Litvinenko's home and at two other London locations he visited on the day he fell ill -- a hotel where he met another ex-KGB spy visiting from Moscow, and a sushi restaurant where he met an Italian academic.
The locations were cordoned off. Police were seen removing material in a metal box from the central London sushi bar.
''POLITICAL PROOCATION'' Putin, meeting EU officials in Helsinki, insisted no evidence linked the Kremlin to the killing.
''It is a great pity that even something as tragic as a man's death is being used for political provocation,'' Putin said. ''I hope the British authorities would not contribute to instigating political scandals. It has nothing to do with reality.'' But the use of such a rare radioactive isotope -- which can be made only in a nuclear reactor -- suggested to experts that only a very sophisticated organisation, if not a powerful state, could be behind the crime.
''This is not some random killing. This is not a tool chosen by a group of amateurs. These people had some serious resources behind them,'' Dr Andrea Sella, lecturer in chemistry at University College London, told Reuters.
Litvinenko, who became a British citizen last month, was one of a group of Putin opponents who have clustered in London, frequently attracting Moscow's scorn.
A Russian ex-spy came forward in Moscow to acknowledge that he was the man who met Litvinenko at a London hotel with another Russian the day he suddenly fell ill.
The man, Andrei Lugovoy, said in a statement they had met to discuss a business project and he had nothing to do with Litvinenko's death.
Reuters BDP VV0859