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LONDON - Doctors treating the former KGB agent Alexander Litvinenko have said he is unlikely to have been poisoned with the lethal toxin thallium and the cause of his symptoms is unknown.
Urgent tests are being conducted to try to identify the substance or substances which have left the 43-year-old intelligence officer in an intensive care unit with a crippled immune system.
In a day of further twists to the Cold War-style saga, which is the subject of an international hunt by Scotland Yard's anti-terrorist branch, experts raised the possibility that Mr Litvinenko had been attacked with a radioactive chemical.
Early tests had indicated that the former agent, a strident critic of Russian President Vladimir Putin, was poisoned on November 1 with a thallium salt, known as the "secret agents' poison" because it is tasteless, odourless and deadly in very small quantities.
But Dr Amit Nathwani, the consultant caring for Mr Litvinenko at University College Hospital in north London, said: "We have requested toxicology tests to establish what poisoned him.
"Based on results we have received today and Mr Litvinenko's clinical features, thallium poisoning is an unlikely cause of his current condition.
"Further tests will be carried out to establish whether or not there is a single cause for Mr Litvinenko's condition."
The specialist added it was possible that a precise cause of the poisoning might never be known and his treatment could last months.
The development raises the prospect that the dissident intelligence officer, who fled Moscow to Britain in 2000, may have been attacked with a highly-sophisticated and previously unknown toxin.
Doubts about the presence of thallium were raised after Mr Litvinenko suffered a collapse of his bone marrow function and infection-fighting white cells, symptoms that are not associated with the poison.
Professor John Henry, a leading toxicologist, who first confirmed the poisoning of the Ukrainian president Viktor Yushchenko with dioxin in 2004, said: "Mr Litvinenko has signs and symptoms which are unexpected and do not normally occur with thallium poisoning, especially the fact that his bone marrow is not functioning and his white cell count has dropped to zero. Something other than thallium is involved."
The academic, based at St Mary's Hospital in west London, who is advising on Mr Litvinenko's treatment, said he believed a radioactive substance, including a radioactive version of thallium, had been swallowed by him.
But it was unclear whether Mr Litvinenko could have swallowed enough of the radioactive thallium 201 to suffer such extreme symptoms as the loss of his bone marrow.
Low doses of thallium 201 are used as a "tracer" in hi-tech scans for heart disease.
Dr Nathwani said it was "speculation" that a radioactive substance was the cause of Mr Litvinenko's condition.
The Independent has been told that his medical team are investigating whether he was given a drug similar to those used in cancer chemotherapy which would be capable of killing off bone marrow.
Professor David Coggon, a specialist in environmental medicine at Southampton University, said: "Radiation doses have to be relatively high to cause short-term toxic effects."
Friends of Mr Litvinenko, a former pentathlete who ran five miles every day, said that his condition had further deteriorated.
Doctors have said he has a 50-50 chance of surviving the next three or four weeks.
Alex Goldfarb, who helped the Russian officer seek asylum in Britain in 2000 after he turned whistleblower on the KGB's successor, the FSB, said: "To me, he looks like he is slowly deteriorating. He is in danger of a sudden heart or kidney failure. We have to wait to see if his immune system responds. Otherwise the last resort is for him to undergo a bone marrow transplant."
London meetings
The former lieutenant-colonel in the FSB's organised crime unit fell ill on November 1 after two meetings in central London, one with an Italian academic and examining magistrate, Mario Scaramella, at a sushi bar in London's Piccadilly.
Mr Litvinenko believes he was poisoned on the orders of the FSB after he made a series of allegations of criminal activity against his former employer, including the murder of hundreds of innocent Russians in apartment block bombs blamed on Chechen separatists.
He also accused the FSB of ordering him to assassinate the oligarch Boris Berezovsky at a time when it was headed by Mr Putin.
The Kremlin has described the claims of its involvement in the poisoning as "sheer nonsense".
Friends of Mr Litvinenko suggest it followed his recording of a tape which supposedly contains damaging allegations against senior Russian figures, including Mr Putin.
Mr Scaramella claimed yesterday that both he and Mr Litvinenko received death threats and had met for 35 minutes to discuss the plausibility of the threats.
The Italian, an expert on the KGB, showed his Russian contact documents which suggest a name for the killer of the dissident journalist Anna Politkovskaya, whose death was being investigated by Mr Litvinenko.
But sources said last night that detectives were paying close attention to an earlier meeting held by Mr Litvinenko on November 1 at a London hotel with Andrei Lugovoy, a former member of the KGB's elite close protection unit who now runs a private security company in Moscow.
Mr Lugovoy, who was the former head of the KGB's 9th Directorate dedicated to safeguarding VIPs and was also an ally of Mr Berezovsky, met Mr Litvinenko with a second man, named only as Vladimir.
The three men drank tea together.
There is no suggestion that Mr Lugovoy, who was the personal bodyguard of the Russian prime minister in the last 1990s, had any knowledge of the poisoning.
But a source said: "Detectives are looking very closely at what happened at this meeting and studying CCTV footage of the other individual. It could be significant in finding an explanation of why Alexander Litvinenko is so ill."
- INDEPENDENT
