"We are sorry to announce that Alexander Litvinenko died at University College Hospital (UCH) at 9:21 (2121 GMT) on the 23rd of November 2006," the spokesman for the hospital said.

 "He was seriously ill when he was admitted to UCH on Friday November 17, and the medical team at the hospital did everything possible to save his life."

 The condition of the 43-year-old had worsened on Thursday, doctors and friends said, as he suffered a heart attack overnight Wednesday, and was fighting for his life.

 Litvinenko, who was being treated under armed guard at the hospital, was a former lieutenant-colonel in Russia's Federal Security Service (FSB) -- the successor to the Soviet KGB. His friends have in recent days blamed Russia for his apparent poisoning.

 Oleg Gordievsky, a former colonel in the KGB who defected to Britain in the mid-1980s, was unequivocal in where he laid the blame, telling the BBC that he was "very angry that the Russian security service was ... so evil".

 "It was a very sophisticated poison developed in the secret laboratories of the KGB," he said.

 In Helsinki, where President Vladimir Putin is to attend an EU-Russia summit on Friday, a source in the Russian delegation said late Thursday, just prior to news of the death: "Of course it's a human tragedy. A person was poisoned. But the accusations against the Kremlin are so incredible, so nonsense-like, so silly, that the president cannot comment."

 London's Metropolitan Police said that "inquiries continue into the circumstances surrounding how Mr Litvinenko ... became unwell" in a statement. They added that the matter was being "investigated as an unexplained death" and not as a murder.

 Litvinenko, a fierce critic of Putin, first began to feel ill on November 1, after having tea with two Russians at a central London hotel, followed by lunch at a London sushi bar with an Italian academic.

 One of the Russians and the Italian have said that they will give statements to British embassies in Moscow and Rome respectively, but police were reportedly searching for the other Russian -- identified only as "Vladimir" -- whom they see as central to their investigation.

 Litvinenko fled Russia and was granted asylum in Britain after accusing the FSB of plotting to kill the exiled Russian businessman Boris Berezovsky. He recently became a British citizen.

 Doctors earlier ruled out an initial theory that the heavy metal thallium was responsible, said radioactivity was "unlikely" and dismissed a report three unidentified objects had been found in his intestines.

Agence France Presse