A former Russian security agent poisoned in London earlier this month has been moved to intensive care after his condition deteriorated, doctors said today, as the Kremlin dismissed allegations that he was attacked on the orders of Moscow.

Alexander Litvinenko, a former lieutenant colonel in the FSB, the successor to the KBG, and a vociferous critic of President Vladimir Putin, fell ill after a series of meetings in Central London on November 1.

His condition has threatened to cause a diplomatic crisis between Britain and Russia after doctors confirmed that he was poisoned. Scotland Yard said that it was investigating the attack and his friends and associates directly accused the Kremlin of attempting to kill him.

Today, University College Hospital said that the dissident, 44, had been moved to intensive care after a "slight deterioration in his condition". Mr Litvinenko was moved from Barnet hospital on Friday after his immune system started to collapse, apparently as a consequence of thallium sulphate poisoning.

Mr Litvinenko's friends say that the formerly healthy officer, who used to run five miles a day, looks "like a ghost". He has lost all his hair, has trouble speaking and is at risk of kidney and heart failure. Doctors say he has a 50 per cent chance of survival and may need a bone marrow transplant.

Speaking outside hospital today, Alex Goldfarb, a friend of Mr Litvinenko who has directly accused the FSB of trying to assassinate his fellow dissident said: "It is a very hard sight to see. Just a month ago he was a fit, vigorous and handsome man who was exercising and running five miles a day but now he is in this condition in hospital fighting for his life."

"Those of us who are critical of the regime felt safe because we felt protected by being on Western soil. But for the first time I feel unsafe."

Mr Litvinenko remains under armed guard and Scotland Yard confirmed today that detectives are making "extensive inquiries", including interviews and analysis of CCTV footage and toxicology reports, to determine how and when he was attacked.

Suspicion among the fellow Russian exiles in London has fallen on a meeting over tea that Mr Litvinenko held on the afternoon of November 1 with two Russian men, one of whom they say was a relatively new acquaintance and former associate of Boris Berezovsky, the billionaire exile and friend of Mr Litvinenko who lives in Surrey.

The FSB has refused to comment on the condition of Mr Litvinenko. The deputy spokesman of the Kremlin, Dmitry Peskov, responded to allegations that the attack was ordered by the Russian Government today saying: "There is no need to comment on statements that are pure nonsense."

The Russian Embassy in London has also denied any involvement in the case and described the illness of Mr Litvinenko as "an accident".

But the police investigation, and accusations of Mr Litivenko's associates and former KGB officers, who say the poisoning carries many of the characteristics of an increasingly active FSB, has the potential to cause the severest test of UK-Russian relations since President Putin came to power six years ago.

Times Recommends

• Zardari still has to tackle tough questions
• Material evidence 'debunks' Turin Shroud
• BBC criticised over Uganda gay execution poll