Britain will be plunged into its worst crisis with Russia since President Putin came to power if a Scotland Yard investigation into the poisoning of a former Russian security agent leads back to the Kremlin, diplomats said last night.

Alexander Litvinenko, said yesterday to look “like a ghost” in hospital, had been targeted and bugged for months by intelligence officers from the Russian Embassy in Kensington, his friends and associates said. His mobile phone calls and e-mails had been intercepted. The Kremlin was accused directly by his associates of being behind an apparent move to eliminate him.

He was poisoned because of his fierce and fearless mockery of President Putin, it was claimed. Oleg Gordievsky, the most senior KGB agent to defect to Britain, told The Times that the attempt to kill Mr Litvinenko was state-sponsored. He insisted that it was carried out by a Russian former colleague who had been recruited in prison by the FSB, the successor to the KGB.

Amid huge concern in diplomatic circles about the accusations of Kremlin involvement, the Foreign Office was awaiting anxiously the results of the police investigation on the apparent attack on a man who is now a British citizen. Scotland Yard said that it was investigating a “suspicious poisoning”.

Mr Litvinenko, 44, is under armed guard in hospital and has only a 50 per cent chance of survival, according to friends. Mr Litvinenko’s wife, Marina, also 44, said: “His bone marrow is destroyed. I may need a donor to save his life.” Doctors last night said that Mr Litvinenko’s bone marrow had failed and was not producing enough white blood cells to maintain his immune system.

He was apparently poisoned with thallium, a highly toxic chemical once used in rat killer, after a secret meeting with an associate in London. Professor John Henry, a leading toxicologist, who examined Mr Litvinenko on Saturday, said he believed that he had been given a potentially lethal dose.

Police want to question two people whom Mr Litvinenko met on the day of the alleged poisoning. One is a Russian man who talked to him over a cup of tea at an hotel and the other is Mario Scaramella, an Italian academic whom he met at a sushi bar.

It emerged last night that before becoming seriously ill, Mr Litvinenko told a journalist contact: “Mario said he wanted to sit down to talk to me so I suggested we go to a Japaneese restaurant nearby.

“I ordered lunch but he ate nothing. He appeared to be very nervous. He handed me a four-page document which he said he wanted me to read straight away. The document was an e-mail. I could not understand why he had come all the way to London to give it to me. Several hours after the meeting I started to feel sick.”

Mr Scaramella visited the British Embassy in Rome a week ago and spoke to staff there once he realised that he was the last person to have seen Mr Litvinenko. Since then he is reported to have gone into hiding, fearing for his life. “Mario is very scared at the moment,” a friend who did not want to give his name said. “He is worried that the Russians and the Chechens are after him. He has obviously been made some sort of scapegoat.”

The Foreign Office said that it had not approached the Russian authorities yet, but was waiting for the outcome of the Scotland Yard investigation. “We will wait until the facts are known,” a spokesman said.

Britain has been involved in a number of diplomatic spats with Russia over the past decade, usually involving espionage. But sources close to the Foreign Office said last night that Mr Litvinenko’s case was far more severe and could lead to the most serious diplomatic incident with Moscow since Mr Putin came to power more than six years ago.

“We are not talking about a routine espionage dispute,” the source said. “This time we are dealing with the attempted murder of a foreign national in a foreign country using methods that we know the Russians are widely capable of. If we get something solid to link this with Moscow it would be taken very seriously. These are the sort of methods normally used by terrorists.”

British intelligence is aware of other thallium attacks elsewhere in Europe involving Russian agents, The Times was told. An official said: “We have to keep an open mind at this stage and not jump to conclusions. It may be that some criminal or other underworld organisation was involved, as opposed to something even more sinister directly linked to the Russian intelligence service.”

Mr Litvinenko was first taken ill on November 1 during a day that included a meeting with Mr Scaramella at a sushi bar in Piccadilly. Mr Scaramella had e-mailed him to say that he had new information on the murder of Anna Politkovskaya, the 48-year-old investigative reporter shot dead in Moscow last month.

Mr Litvinenko ordered the food and the Italian handed over documents that mentioned people who could be linked to the reporter’s murder.The Russian told friends that he felt the first effects of the poison when he got home because he fell down. Friends believe the meeting was genuine. There is no suspicion that the restaurant was involved.

Mr Gordievsky revealed that he had warned Mr Litvinenko on several occasions that he was risking his life by taking such a public anti-Moscow stance. “But he always said to me that he would know whenever he faced the enemy and that he would be able to deal with it,” Mr Gordievsky said.

Alex Goldfarb, a friend of Mr Litvinenko, and a leading figure in a Russian civil rights organisation in New York, said: “He looks terrible. He looks like a ghost. He hasn’t eaten for 18 days. He looks like an old man . . . a month ago he was a fit, handsome, young man.”

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