British police are investigating the near-fatal poisoning of a former Russian spy who has been an outspoken critic of the Kremlin and of his former colleagues in Russia's security agency. Col. Alexander Litvinenko, a former KGB and Federal Security Service (FSB) agent, was under armed guard at a London hospital. He was in a "serious but stable" condition, the hospital said. "He is still very weak," friend Alexander Goldfarb told reporters outside the hospital Sunday. "He is in a fighting mood, though."' Police said a specialist crime unit began an investigation Friday into how Litvinenko may have been poisoned. No arrests had been made, said a Scotland Yard spokesman, speaking on condition of anonymity in line with force policy. Litvinenko, who had been looking into the killing of Russian investigative journalist Anna Politkovskaya, told reporters earlier this week that he fell ill Nov. 1 following a meal at a sushi restaurant with a contact who claimed to have details about the murder. British news outlets, including Sky News and The Independent, identified the contact as Mario Scaramella, an Italian academic who has helped investigate KGB activity in Italy during the Cold War. Scaramella could not immediately be reached for comment. Politkovskaya, who had written critically about abuses by Russian and pro-Moscow Chechen forces fighting separatists in Chechnya, was gunned down Oct. 7 inside her Moscow apartment building. Her attackers have not been found. Rights groups have said the killing underscores the risks faced by Russians who question or criticize the government. A doctor treating Litvinenko told the British Broadcasting Corp. that tests showed he had been poisoned by thallium - a toxic metal found in rat poison. "He's got a prospect of recovering, he has a prospect of dying," said Dr. John Henry, a clinical toxicologist who treated Ukrainian President Viktor Yushchenko when he was poisoned by dioxin during his 2004 presidential election campaign. Henry told the BBC that thallium can cause damage to the nervous system and organ failure, and that just one gram can be lethal. Friends visiting Litvinenko in hospital said they were shocked by his appearance. "He looks like a ghost," said Goldfarb. "He's a very fit man, he never smoked, he never drank, he would run five miles a day, but now he has lost all his hair, he has inflammation in the throat, so he cannot swallow." Litvinenko quit Russia for Britain six years ago and has been an outspoken critic of the Kremlin ever since. In 2003 he wrote a book, "The FSB Blows Up Russia," accusing his country's secret service agency of staging apartment-house bombings in 1999 that killed more than 300 people in Russia and sparked the second war in Chechnya. His friends have said they believe Russian authorities could be behind the poisoning. Moscow did not comment on the allegations. Russian dissident and tycoon Boris Berezovsky, who was at Litvinenko's bedside on Friday, told The Associated Press he suspected Russia's intelligence services were behind the alleged assassination attempt. "It's not complicated to say who fights against him," Berezovsky said in a telephone interview. "He's (Russian President Vladimir) Putin's enemy, he started to criticize him and had lots of fears." Goldfarb, who masterminded Litvinenko's 2000 emigration to Britain, said FSB agents had made death threats against Litvinenko. In an interview given to The Sunday Times newspaper before his condition worsened, Litvinenko described how he had lunch with an Italian contact who claimed to have information on Politkovskaya's killing. "They probably thought I would be dead from heart failure by the third day," the newspaper quoted Litvinenko as saying. "I do feel very bad. I've never felt like this before - like my life is hanging on the ropes." Glenn Edwards, operations manager at Itsu sushi restaurant where Litvinenko had dined, told the AP that detectives had arrived at the restaurant Saturday asking for closed circuit television footage. He added there was no link between the restaurant's food and the poisoning. Some analysts said the alleged attempt on Litvinenko's life bears the hallmarks of a Russian organized operation. Kremlin critics claim poisoning - which is extremely hard to prove - is a common Soviet-era practice that seems to have reappeared since Putin, an ex-KGB officer, became president. Litvinenko joined the KGB counterintelligence forces in 1988, and rose to the rank of colonel in its successor, the Federal Security Service, or FSB. He began specializing in terrorism and organized crime in 1991, and was transferred to the FSB's most secretive department on criminal organizations in 1997. He fled Russia and claimed asylum in Britain in November 2000, two years after publicly accusing his FSB superiors of ordering him to kill Berezovsky, at the time a powerful Kremlin insider. Berezovsky told the AP in telephone interview that Litvinenko fell out with his superiors after he exposed corruption within FSB ranks. In 1999 and 2000, Litvinenko spent nine months in jail awaiting trial on charges of abusing his office, but was acquitted. He then fled to Britain.