UK police are investigating the poisoning of a Russian former security agent and critic of President Vladimir Putin living in exile in Britain.
Alexander Litvinenko is a critic of Vladimir Putin
Alexander Litvinenko, a former colonel in the KGB, told the BBC he fell ill after meeting a contact at a London sushi bar on 1 November.
Police say the 43-year-old is in a serious but stable condition in University College Hospital, London.
Clinical experts said he was poisoned with the highly-toxic metal thallium.
Mr Litvinenko fled Russia and was granted political asylum in Britain in 2001.
It is reported he was granted British citizenship this year, although this has not been confirmed by the Home Office which does not comment on individual cases.
He said he had been investigating the murder of Russian journalist Anna Politkovskaya, who was killed in Moscow last month.
Speaking to the BBC last week, before his condition deteriorated, Mr Litvinenko said the contact had approached him to say they should talk and they arranged to meet at the restaurant in Piccadilly.
"He gave me some papers which contained some names on it - 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."
It was two weeks later that Mr Litvinenko took seriously ill and he was admitted to hospital.
Clinical toxicologist John Henry, who examined Mr Litvinenko on Saturday, told the BBC he believes he was given a potentially lethal dose of thallium.
Describing thallium, he said it 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."
He described Mr Litvinenko's condition as serious.
"I can tell you he's ill. He is quite seriously sick. There's no doubt that he's been poisoned by thallium, and it probably dates back to 1 November, when he first started to get ill."
Mr Litvinenko's friend Alex Goldfarb, who has been visiting him in hospital, said: "He looks terrible. He looks like a ghost actually. He lost all his hair. He hasn't eaten for eighteen 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."
There has been no comment from the Kremlin and the Russian media is reportedly keeping quiet on the incident.
Scotland Yard confirmed police were investigating, saying there had been no arrests but inquiries were continuing.
Mr Litvinenko had earlier alleged that members of the Federal Security Service (FSB) - the main successor to the Soviet KGB - had plotted to kill the powerful Russian tycoon Boris Berezovsky.
He also wrote a book called Blowing up Russia: Terror from Within, alleging that FSB agents coordinated 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.
Anna Politkovskaya once fled to Austria after receiving threats
Ms Politkovskaya, a harsh critic of Mr Putin and Russian policy in Chechnya, was shot dead at her Moscow apartment building.
She was one of the few Russian journalists to write about alleged human rights abuses in Chechnya and had received death threats in the past.
Ms Politkovskaya became ill with food-poisoning on her way to report on the Beslan school siege in 2004, which some believed may have been an attempt on her life.
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