The former Russian spy Alexander Litvinenko died in hospital last night after his condition rapidly deteriorated following a heart attack and cardiac failure.
Mr Litvinenko, 43, once a KGB officer and latterly a fierce critic of President Vladimir Putin, had been on a ventilator and unconscious for most of yesterday while being treated for suspected poisoning at University College London Hospital.
Doctors said they had done everything they could to save his life but had struggled as his condition deteriorated. A spokesman for UCLH said Mr Litvinenko had died at 9pm. He added: "Every avenue was explored to establish the cause of his condition. Our thoughts are with Mr Litvinenko's family."
Just before Mr Litvinenko lapsed into unconsciousness yesterday he spoke of his determination to survive, but insisted that the campaign for truth would go on with or without him. "I want to survive just to show them ... the bastards got me, but they won't get everybody ... this is what it takes to prove one has been telling the truth."
Andrei Nekrasov, a friend and film-maker who visited him each day this week, said: "The figure who greeted me looked like the survivor of a Nazi concentration camp. We discussed the likelihood of another killing. Sasha [Litvinenko] warned me not to go back to Russia because it was too dangerous. Very sadly, he turned out to be the next victim, attacked in the perceived safety of central London." He described Mr Litvinenko as too weak to move his limbs and visibly in great pain.
Doctors are still trying determine what caused Mr Litvinenko's health to fail so quickly. Yesterday they dismissed reports of him ingesting three mysterious dark objects, discounting X-rays which showed the "dense matter" in his intestines. Dr Geoff Bellingan, the director of critical care at UCLH, said: "Suggestions that an X-ray identified three objects in his body are misleading. We are now convinced that shadowing on the X-ray was caused by Prussian Blue - a non-toxic therapeutic agent which was administered as part of his treatment. We are still unclear as to the cause of his condition."
Mr Litvinenko, who believes he was the victim of an assassination attempt sanctioned by Moscow, was at first thought to have been poisoned by thallium, a colourless and odourless heavy metal. This was ruled out and doctors then thought the cause was a radioactive substance, possibly thallium 201, which is used in hospitals as a tracer in cardiac tests. Yesterday however this was also said to have been "unlikely". By the end, his bone marrow had badly deteriorated, leaving him with no immune system, and his liver had been destroyed.
Last night, Oleg Gordievsky, a British double agent smuggled out of Russia by a British spy in 1985, said: "It was very sad because he was a hero of Russia. He was a hero of Great Britain. He said what a lovely country Britain was... he was fighting against the evil forces of Russia."
Mr Litvinenko, who defected six years ago and recently became a British citizen, had been investigating the recent murder of the Russian journalist Anna Politkovskaya when he fell ill. She had worked to uncover the truth about torture in Chechnya.
A Scotland Yard spokesman said yesterday "the matter is being investigated as an unexplained death". The anti-terrorist branch is looking at two meetings on 1 November, the day Mr Litvinenko was poisoned. One was with two Russians at a London hotel and the second was with an Italian espionage expert, Mario Scaramella, at the Itsu sushi restaurant on Piccadilly.
Mr Litvinenko told friends from hospital that he had tea at the hotel with Andrei Lugovoy, a former member of the KGB with links to the London-based billionaire Boris Berezovsky and another man, who has not been named. Later that day Mr Litvinenko, met Mr Scaramella who passed on information about who had gunned down Politkovskaya. The next day Mr Litvinenko called an ambulance and was taken to Barnet Hospital. He deteriorated and he was moved to UCLH on 17 November.
Mr Litvinenko's gravest allegation is made in a book he wrote in 2003, Blowing Up Russia, in which he accused the Russian secret service of planting bombs which triggered the second Chechen war.