There has been a "dramatic deterioration" in the condition of former Russian spy Alexander Litvinenko, doctors have said.
There is confusion over what has poisoned the ex-spy
Mr Litvinenko, 43, is now critically ill in intensive care at University College Hospital in London, after suffering a heart attack overnight.
Critical care head Geoff Bellingan said it was unlikely the ex-KGB agent was poisoned by thallium.
The Kremlin and Russian security services have denied any involvement.
Friends of Mr Litvinenko have claimed he was poisoned in London earlier this month because of his criticism of Russian President Vladimir Putin.
Dr Bellingan said he was "concerned by speculation in the media by people who are not directly involved" in the care of Mr Litvinenko.
Cause still unclear
Initial reports said Mr Litvinenko was poisoned with the heavy metal thallium, and the theory that some form of radioactive material was used has also been suggested.
But Dr Bellingan dismissed both of these explanations.
Reports of three objects found on X-rays of the patient were "misleading" and were almost certainly shadows caused by Prussian Blue, used to treat thallium or caesium poisoning, he said.
"Despite extensive tests, we are still unclear as to the cause of his condition," Dr Bellingan added.
Police, who have said previously they suspected "deliberate poisoning", said there had been no significant developments in the case.
Investigations are examining two meetings, one at a London hotel with a former KGB agent and another man, and another with Italian security consultant Mario Scaramella at a sushi restaurant in London's West End.
Mr Litvinenko's friend Alex Goldfarb said: "I have seen Alexander through the window, he is heavily sedated, and he is on a ventilator because overnight he went into a heart failure, although the doctor said that his heart is not damaged.
"The situation could get better but his chances are obviously decreased now because while he is on this ventilator machine it would be much more difficult to speak about his bone marrow transplant which until today was a major problem for the doctors."
Mr Litvinenko fled to the UK in 2000, claiming persecution in Russia, and was granted asylum. He is understood to have taken British citizenship this year.
Both the Kremlin and Russia's foreign intelligence service, the SVR, have denied any part in poisoning Mr Litvinenko, who is a former security agent with Russia's Federal Security Service (FSB).