Alexander Litvinenko died in a London hospital last night three weeks after succumbing to a mysterious poison he said was administered by his former colleagues in Russian intelligence.

University College Hospital said that the former Russian intelligence officer died after suffering a heart attack on Wednesday night when he fell into unconsciousness and was put on a life-support machine.

His death threatens to plunge relations between Britain and Russia into crisis. The anti-terrorist squad at Scotland Yard were leading the investigation into his suspected poisoning, which is now likely to become a murder inquiry.

President Putin of Russia could be questioned in public about the death today, when he attends an EU-Russia summit in Helsinki. He may be asked to respond to allegations that members of his security services were responsible for killing Mr Litvinenko, an outspoken critic of the regime.

In his last interview, Mr Litvinenko made clear that he blamed the Kremlin for his condition but appeared determined that opposition to Mr Putin continues.

A spokesman for University College Hospital, where Mr Litvinenko died at 9.21pm, said that the medical team had done everything possible to save his life.

The spokesman added: “Every avenue was explored to establish the cause of his condition and the matter is now an ongoing investigation being dealt with by detectives from New Scotland Yard. Because of this we will not be commenting any further on this matter. Our thoughts are with Mr Litvinenko’s family.”

Scotland Yard confirmed last night that they were treating it as an unexplained death.

The 43-year-old dissident was put on a ventilator yesterday after suffering heart failure. A further battery of tests yesterday ruled out thallium as the cause and doctors have now also dismissed radiation poisoning. As speculation intensified, on both the identity of the would-be assassin and the nature of the poison, doctors were forced to deny reports that three objects had shown up in X-rays of the patient.

Geoff Bellingan, head of critical care at UCL, said that shadows seen on the X-rays were caused by “Prussian blue”, which is used to treat thallium or caesium poisoning.

One theory was that some sort of chemotherapy drug had been used against Mr Litvinenko, who was given British citizenship after he fled Moscow in 2001.

Investigators said yesterday that without further help from Mr Litvinenko in recalling who he had met and what he had eaten and drunk in the 72 hours before he fell ill, their efforts to find the assassins would be hgreatly ampered.

Boris Berezovsky, the Russian multimillionaire who blames the Kremlin for the attack and had visited Mr Litvinenko in hospital before his death, said yesterday that he hoped that counter-terrorist detectives would “interview me soon about the poisoning”.

Denis MacShane, the former Foreign and Commonwealth Office minister, said that he was alarmed by claims about a Russian plot and said that there were “too many other examples” to dismiss the allegations. But he told Channel 4 News: “I think we shouldn’t rush to judgment.”

Claims surrounding the affair were “an all-out war between some incredibly powerful, rich, very well-connected people — some of them based in London — with great international contacts, and the Putin Government,” he added.

Mr MacShane said: “It should be left to British authorities, British doctors and British policemen” to say what happened. He described some of the press coverage as ghoulish, but Mr Litvinenko’s friends said that he had been pleased with the publicity, which he believed had raised questions about Russian hit squads operating abroad.

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