Scotland Yard is investigating the attempted murder of a top Russian defector poisoned by political enemies in London. Alexander Litvinenko, an ex-KGB colonel who fled the current Russian regime to claim asylum in Britain, is under armed police guard in hospital.
Sources have confirmed that the Russian was taken suddenly and dangerously ill on November 1 while investigating the recent murder of dissident Russian journalist Anna Politkovskaya. Mr Litvinenko was poisoned following a clandestine meeting with an associate at a sushi bar in London's Piccadilly.
The ex-KGB man was given documents which claimed to name Ms Politkovskaya's killers. According to the papers, she was murdered by four members of President Vladimir Putin's federal security service, known as the FSB. A source close to Mr Litvinenko claimed he had been the victim of a revenge attack by the increasingly hard-line Russian regime.
The source added: "He is convinced that he has been poisoned at the instigation of President Putin."
Any suggestion that Putin's men are attacking their enemies on British soil is bound to place serious strain on relations between the two countries - and raises the ghosts of Cold War scandals such as the assassination of Georgi Markov, the Bulgarian dissident murdered with a poison-tipped umbrella.
Scotland Yard confirmed last night that the Litvinenko case was being investigated as a 'suspicious poisoning' and that his condition was 'serious, but stable'.
It was also clear that MI5 had launched an urgent operation.
Senior security sources told The Mail on Sunday that the Russian had been poisoned with thallium, a virulent toxin that can cause death within ten hours.
Mr Litvinenko collapsed three weeks ago after meeting an Italian associate, Mario Scaramella, at the Itsu Japanese restaurant in Piccadilly.
Friends of the Russian believe he was followed around London by the FSB, which also monitored his e-mails and phone calls. Putin's agents would then have seen him take receipt of the documents at the sushi bar - and decided to move in for the kill.
Thallium - an odourless, colourless poison - can be lethal even in doses of less than a gram. Mr Litvinenko's supporters believe FSB agents injected the toxin into a meal either at the restaurant or shortly after his meeting with Mr Scaramella.
The Russian and his supporters are certain Mr Scaramella had no knowledge of the poison attempt.
The victim survived, they say, only because of his high fitness levels and his quick-wittedness when he first felt ill. He induced vomiting in an attempt to rid his system of the poison.
He told the Russian Izvestia news agency last week that Mr Scaramella had e-mailed him from Italy in October asking to meet him in London on November 10 and 11.
Mr Litvinenko added: "But suddenly he called me on November 1 and, as usual, we decided to meet at Piccadilly Circus. We met at around 3pm and I invited him to dine in the restaurant.
"I ordered the food, and he took just water and was hurrying me. From the text of the documents, I understood that the mentioned people could have arranged the murder of Anna Politkovskaya. We parted nearly at once. As soon as I got home, I fell down."
Last night Tatiane Assis, duty manager of the Itsu sushi bar, said: "The police came here in the morning and they asked if we had CCTV cameras around. They didn't give me any details - they just said it was because they were looking into a poisoning but they weren't sure if it was in here. They didn't take anything away and we don't have any CCTV."
Mr Litvinenko, a former colonel in the FSB and before that its predecessor the KGB, defected to Britain in 2000 after fleeing treason charges in Russia. He was granted political asylum in May 2001.
Last night, friends of Mr Litvinenko said he had lost his hair, had difficulty speaking and had a 50 per cent chance of survival in University College Hospital, London.
Mr Litvinenko has proved a constant irritant to Mr Putin since a meeting between the pair eight years ago.
In 1998, shortly after Mr Putin was appointed head of the FSB, he invited Mr Litvinenko to meet him. Mr Litvinenko has described how he used the occasion to tell Mr Putin about corrupt practices in the organisation. The meeting was cut short and he was suspended.
In November that year he went public with his claims - including the bombshell allegation that the FSB had plotted to kill the business tycoon Boris Berezovsky. From that point on, Mr Litvinenko was in and out of jail, facing corruption charges, until he fled to Britain.
The following year he co-authored a book - apparently funded by Berezovsky - Blowing Up Russia: Terror From Within, which claimed the Russian security services had used organised crime gangs and war criminals to carry out contract killings in Russia and abroad.
He described a secret FSB department which specialised in locating and liquidating people considered a danger to the state. He also alleged that top Russian officials took million-dollar payments from Chechen leaders as payment for weapons and ammunition left in the war-ravaged republic by Russian troops - and in exchange for Russian commanders agreeing to halt certain military operations.
Most damagingly, he claimed that the FSB was behind the deadly apartment bombings in Moscow in 1999, which he said the Kremlin had blamed on Chechen terrorists. Recently he had become interested in the murder of Anna Politkovskaya, a journalist who investigated human-rights abuses by Russians in Chechnya and was gunned down in her Moscow apartment building in October.
The poisoning mystery recalls the spy dramas of the Cold War, when deep mistrust existed between East and West. The era was synonymous with treachery and betrayal - and some would go to almost any lengths to silence opponents.
One notorious case was that of Georgi Markov, the Bulgarian dissident killed by a poisoned umbrella in 1978.
He was standing at a bus stop on Waterloo Bridge when he felt a sting in his right leg. At the same moment, a man behind him picked up an umbrella he had dropped and sprinted away to get into a taxi.
Three days later, Markov was dead, a victim of the Cold War and the Bulgarian secret police.
Two years ago the world was given a startling reminder of the Soviet Union's grisly heritage with the attempted murder of Ukrainian presidential candidate Viktor Yushchenko. He was poisoned by political rivals, and his face is still grossly disfigured.
Professor Scaramella, who Mr Litvinenko met in the sushi bar, is an academic at Naples University and a consultant to the Mitrokhin Commission which was set up by the Italian parliament to investigate the activities of the KGB in Italy during the Cold War.
In 2004 he was the victim of a murder attempt when a Mafia hitman tried to kill him. Several shots were fired but the gunman missed.