UK police are investigating after a Russian former security agent in exile in Britain was poisoned by thallium.
Alexander Litvinenko is a critic of Vladimir Putin
Alexander Litvinenko, a former KGB colonel and critic of President Vladimir Putin, fell ill on 1 November after a meeting at a London sushi bar.
A clinical toxicologist said the 43-year-old had been poisoned with a potentially lethal dose of the metal.
Mr Litvinenko is in a serious but stable condition in University College Hospital, London.
He is reported to be under armed guard.
Mr Litvinenko had been investigating the murder of Russian journalist Anna Politkovskaya, a harsh critic of Mr Putin and Russian policy in Chechnya, who was killed in Moscow last month.
Speaking to the BBC last week, he said a contact had approached him to say they should talk, and they arranged to meet at a restaurant in Piccadilly.
"He gave me some papers which contained some names - perhaps names of those who may have been involved in the murder of Anna Politkovskaya - and several hours after the meeting I started to feel sick."
Two weeks later Mr Litvinenko was taken seriously ill and admitted to hospital.
Clinical toxicologist John Henry said Mr Litvinenko was "quite seriously sick" and there was "no doubt" he had been poisoned by thallium, probably on 1 November.
He said thallium was a "little bit like table salt" and that a very small amount could be lethal.
"It is tasteless, colourless, odourless. It takes about a gram - you know, a large pinch of salt like in your food - to kill you."
Mr Litvinenko's friend Alex Goldfarb, who has been visiting him in hospital, said doctors told him he had a 50/50 chance of surviving the next three or four weeks.
"He looks like a ghost. He lost all his hair. He hasn't eaten for 18 days. He looks like an old man... a month ago he was a fit handsome young man."
He added: "I think this is the work of the Russian Secret Service."
Dr Andres Virchis, a doctor from Barnet Hospital who earlier treated Mr Litvinenko, said his bone marrow had failed and he was not producing any normal immune cells.
That was "presumably as the effect of the thallium or even potentially some other unknown substances that we're not aware of", he said.
Russian tycoon Boris Berezovsky, who also lives in Britain, said thallium was a "special" poison, that "you couldn't just get over the counter".
"You could say it is only available to secret services," he said.
There has been no comment from the Kremlin and the Russian media is reportedly keeping quiet on the incident.
Scotland Yard said there had been no arrests but inquiries were continuing. Officers questioned Mr Litvinenko in hospital for several hours on Sunday.
Mr Litvinenko fled Russia and was granted political asylum in Britain in 2001.
He is believed to have taken British citizenship this year, although this has not been confirmed by the Home Office.
Journalist Anna Politkovskaya was shot dead earlier this year
Oleg Gordievsky, another former KGB colonel who defected to the UK and knows Mr Litvinenko well, said: "This assassination attempt was an attempt to kill a British citizen on British soil. It is absolutely outrageous."
He told the BBC he believed Mr Litvinenko was poisoned when he drank a cup of tea at the flat of an old Russian friend - before the lunchtime meeting at the sushi restaurant.
In a book, Blowing up Russia: Terror from Within, Mr Litvinenko alleged that agents from KGB successor FSB co-ordinated the 1999 apartment block bombings in Russia that killed more than 300 people.
Russian officials blamed the explosions on Chechen separatists and in that year the Kremlin launched a new military offensive on Chechnya.