The dissident former Russian spy who accused Vladimir Putin of having him poisoned was killed by a massive dose of a deadly radioactive element, it was disclosed today.
A large quantity of alpha radiation from polonium 210 was detected in Alexander Litvinenko's urine, apparently just a few hours prior to his death in University College Hospital last night.
Tonight it emerged that experts had discovered traces of radiation at the two premises - a hotel and sushi bar - which Mr Litvinenko visited on November 1, the day he fell ill.
The Health Protection Agency attempted to calm public anxiety, saying the risk of contamination was low, but it admitted that the apparent poisoning was an "unprecedented" situation.
The disclosure that radiation was to blame for Mr Litvinenko's death came after his family released his deathbed statement, in which the Kremlin critic and former security agent pointed the finger of blame at Russian president Mr Putin.
"You have shown yourself to be as barbaric and ruthless as your most hostile critics have claimed," he said.
"You may succeed in silencing one man, but the howl of protest from around the world will reverberate, Mr Putin, in your ears for the rest of your life.
"May God forgive you for what you have done, not only to me, but to beloved Russia and its people."
Mr Litvinenko's tearful father Walter blamed the Russian security services for the death of his "courageous" son.
The ex-spy's statement prompted Mr Putin to speak out for the first time about the affair. He insisted there was no proof that Mr Litvinenko's death was "violent".
The day of drama came at the end of an extraordinary week in which the Cold War-style saga has taken several twists.
Scotland Yard's counter terrorism unit, led by Deputy Assistant Commissioner Peter Clarke, was tonight heading an investigation into Mr Litvinenko's poisoning.
And senior Government ministers and security officials held a meeting of Cobra, the Government's emergency planning committee.