Doctors treating a former Russian spy and fierce Kremlin critic who is fighting for his life in a London hospital said Tuesday that test results showed it was unlikely he had been poisoned by the toxic metal thallium, as previously thought.

University College Hospital said doctors were looking at other possible causes for Col. Alexander Litvinenko's illness.

"Based on results we have received today and Mr. Litvinenko's clinical features, thallium poisoning is an unlikely cause of his current condition," the hospital said.

Dr. Amit Nathwani, a member of the team treating the former KGB officer, said "the levels of thallium we are able to detect are not the levels we expect to see in toxicity."

Doctors had earlier identified thallium — a colorless, odorless heavy metal lethal in doses of as little as one gram — as the likely cause of Litvinenko's illness.

But Dr. John Henry, who also was treating Litvinenko, said Tuesday that a radioactive substance may have been involved.

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Henry offered a number of possibilities to explain Litvinenko's symptoms, which include hair loss and damage to the immune system and vital organs.

Henry had said Litvinenko may have been given thallium alongside a second drug or in combination with a radioactive substance, or perhaps was poisoned with radioactive thallium. "At this stage, radioactive thallium seems the most likely cause," he said.

The Metropolitan Police, whose anti-terrorist unit is leading the investigation into Litvinenko's poisoning, said they were continuing to piece together the 43-year-old's movements on Nov. 1, the day he fell ill.

Litvinenko remained under armed guard Tuesday, the victim of what his friends and fellow dissidents called an assassination attempt by the Russian government and FSB, which he has accused of human rights abuses and corruption.

The Kremlin and Russia's foreign intelligence agency have strongly denied any involvement in the attack on Litvinenko.

On the day he claimed to have been poisoned, he met with two Russians before dining at a sushi restaurant with Italian security expert Mario Scaramella. Scaramella said Tuesday he had met Litvinenko to show him e-mails from a confidential source identifying the killers of investigative journalist Anna Politkovskaya, another high- profile Kremlin critic, who was gunned down Oct. 7 at her Moscow apartment building.

Doctors had earlier given Litvinenko a 50 percent chance of survival. Henry said Tuesday the former agent was able to eat and to talk. "At the moment he's not getting better, but he's holding up," he said.

Radioactive thallium is commonly used to measure blood flow imaging in hospitals, but not in high doses, Henry said.

Although he refused to speculate about whether Litvinenko was deliberately poisoned, Henry explained that "poisons can be taken by mouth, they can be injected, they can be inhaled."

"In this case, his symptoms are gastrointestinal to start with. He had gut problems, so the probability is that he swallowed something that was poisonous," Henry said, adding that the toxin would put his bone marrow at an extremely high risk and attack the body's cells.

Litvinenko left Russia for Britain six years ago and has been an outspoken critic of the Kremlin ever since, most recently investigating the death of Politkovskaya.

"Somebody has asked me directly, who is guilty of Anna's death? And I can directly answer you: It is Mr. Putin, president of the Russian Federation," Litvinenko told a videotaped meeting discussing Politkovskaya's death at the Frontline media club in London in October.

Kremlin spokesman Dmitry Peskov dismissed suggestions that Russian intelligence services were involved as "nothing but sheer nonsense."

Litvinenko joined the KGB counterintelligence forces in 1988, and rose to the rank of colonel in the 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.

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.

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