In Moscow, Russian intelligence denied any involvement in the poisoning of Alexander Litvinenko, a former lieutenant colonel in Russia's Federal Security Service (FSB), the successor to the Soviet KGB, who fell ill November 1.
"We have nothing to do with what happened to Litvinenko. The Russian secret services have not in a long time carried out poisonings or any form of assassination," said a spokesman for the foreign intelligence service, the SVR.
In London's University College Hospital, doctor Stephen Rowley said the Russian remained "in a serious condition. Last night, due to a slight deterioration in his condition he was transferred to the intensive care unit as a precautionary measure."
Alexander Goldfarb, a friend who helped Litvinenko defect to Britain and become a British citizen a month ago, found him "more tired today, more exhausted" during a hospital visit on Monday.
He has said the once-fit man looks like a "ghost" and a cancer patient who has lost his hair.
London's Metropolitan Police said they were investigating "a suspected deliberate poisoning" after receiving information on November 17 about a patient who was in serious but stable condition.
The police did not name the subject but British media said they were referring to Litvinenko.
In trying to determine how he became ill, police were "interviewing possible witnesses -- including the man -- and examining his movements in and around the time of the suspected poisoning" as well as closed-circuit television footage.
They said they were also awaiting the results of toxicology tests.
Goldfarb and other friends of Litvinenko suspect that the FSB was out to get the outspoken defector, who was granted political asylum in Britain in 2001.
"It's linked to Moscow and the FSB," Goldfarb said, adding that Litvinenko had written two books accusing the Russian secret services of criminal activities. "He was a very vocal critic of President Putin."
Oleg Gordievsky, a former KGB station head in London and a close friend of Litvinenko, told The Times newspaper that the poisoning was "state-sponsored" and "only" Russian intelligence could have done it.
Gordievsky, the highest-ranking Soviet defector ever, said "he was such an obvious enemy ... The poison was very sophisticated."
Britain's domestic Press Association news agency reported that Italian academic Mario Scaramella, who was Litvinenko's last contact before falling ill, went into hiding a week ago.
The Sunday Times said that over lunch last month in a London sushi bar, Scaramella gave Litvinenko a document with information on the murder last month of Russian journalist Anna Politkovskaya, reportedly a friend of his.
But Gordievsky dismissed any possibility that Scaramella could be involved in the poisoning, saying that Litvinenko "was already feeling unwell before the lunch (with Scaramella). He was poisoned before he met the Italian."
Goldfarb said Litvinenko had confirmed to him that he had met two Russian men for tea in a hotel in central London "briefly" before his meeting with Scaramella.
The Russian embassy in London was unavailable for comment.
Sources close to the Foreign Office told The Times that if Russia were responsible, "it would be taken very seriously" as it amounted to an attempted murder of a foreign national using "methods normally used by terrorists."
A spokesman for the Foreign Office said that it had not yet contacted Russia over the matter and would follow the advice of the Metropolitan Police.
If Russia's security services were behind the alleged poisoning, it would not be the first time that they have tried to silence critics on the streets of London.

Copyright AFP (Agence France-Presse), 2006