Former Russian spy Alexander Litvinenko, suspected of having been poisoned by his country's security services, died last night.
The ex-KGB officer had been fighting for his life after suffering a heart attack.
Doctors and police conducted further tests yesterday as they tried to trace what made him ill.
His friends believe he was poisoned by Russian security services because of his fierce criticism of President Vladimir Putin's regime - the Kremlin denies the claim.
A spokesman for London's University College Hospital said Mr Litvinenko died at 9.21pm despite the medical team's best efforts.
He said: "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.
"Our thoughts are with Mr Litvinenko's family."
Police are investigating how he became ill and have said that they suspect "deliberate poisoning".
Mr Litvinenko, a defector to Britain who was granted asylum and citizenship, is thought to have been poisoned on November 1.
He had been investigating the murder of Russian investigative journalist Anna Politkovskaya.
In his final interview, hours before he fell unconscious, he remained defiant against the people he believed had poisoned him.
"I want to survive, just to show them," he told filmmaker and friend Andrei Nekrasov.
Mr Nekrasov told The Times that Mr Litvinenko had said: "The bastards got me but they won't get everybody,"
Scotland Yard's counter-terrorism unit is heading the investigation into the alleged poisoning.