A former Russian KGB colonel living in Britain and poisoned by the toxic chemical thallium has returned to intensive care, doctors have said.
Alexander Litvinenko, 43 - a vocal critic of Russian President Vladimir Putin - fell ill on 1 November after a meeting at a London sushi bar.
He is in a serious but stable condition in University College Hospital, London.
Doctors say he was moved to intensive care as a precaution, after his condition deteriorated slightly.
Clinical toxicologist John Henry has said he was poisoned with a potentially lethal dose of the metal thallium.
Alex Goldfarb, who has been visiting Mr Litvinenko in hospital, said his friend had been poisoned because he was critical of the Russian government.
Mr Goldfarb added that doctors had told him he had a 50/50 chance of surviving the next three to four weeks.
He told BBC Radio 4's Today programme: "Of course we do not have any direct evidence other than he met some people during that day.
"He actually had a couple of meetings where he had drinks and this poison could be sprinkled there."
Mr Litvinenko had been investigating the murder of Russian journalist Anna Politkovskaya, an outspoken critic of Mr Putin and Russian policy in Chechnya, who was shot dead at her Moscow apartment building 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.
Mr Henry said Mr Litvinenko was "quite seriously sick" and there was "no doubt" he had been poisoned by thallium, probably on 1 November.
"It is tasteless, colourless, odourless. It takes about a gram - you know, a large pinch of salt like in your food - to kill you", 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.
Russian secret service
BBC security correspondent Gordon Corera says while there is no confirmed link to the poisoning, the Russian secret service has been accused in the past of carrying out operations against dissidents in a number of countries.
There has been no comment from the Kremlin.
Journalist Anna Politkovskaya was gunned down in Moscow last month
Scotland Yard has confirmed it is investigating a "suspected poisoning", but there have been no arrests and inquiries are continuing.
In a book, Blowing up Russia: Terror from Within, Mr Litvinenko alleged that agents of the KGB's successor, the Federal Security Service (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.