LONDON — The hospital treating a former Russian spy and fierce Kremlin critic fighting for his life in an intensive care unit said yesterday that test results showed it was unlikely that he had been poisoned by the toxic metal thallium as previously thought.While doctors said it was clear Alexander Litvinenko had been poisoned, University College Hospital said doctors were looking at substances other than thallium that might have caused his 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 in a statement.

However the hospital said that while preliminary tests suggested Litvinenko had not been poisoned by thallium salt — the most common form of the toxin — radioactive thallium remained a possibility.

Dr. Amit Nathwani, one 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 colourless, odourless heavy metal lethal in doses of as little as one gram — as the likely cause of Litvinenko’s illness. But Dr. John Henry, an eminent toxicologist who has visited Litvinenko several times in hospital, said that a radioactive substance may have been involved.

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

Henry said Litvinenko may have been given thallium alongside a second drug or in combination with a radioactive substance, or poisoned with radioactive thallium.

“At this stage radioactive thallium seems the most likely cause,” he said before test results were complete.

The Metropolitan Police, whose anti-terrorist unit is leading the investigation into Litvinenko’s poisoning, said in a statement that they were continuing to piece together the 43-year-old’s movements on November 1, the day he became ill. The force refused to speculate on the cause of his illness.

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

On the day he claims to have been poisoned, he met with two Russian contacts before dining at a sushi restaurant with Italian security expert Mario Scaramella. Scaramella said yesterday 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 on October 7 inside her Moscow apartment building.

Scaramella said the e-mails listed other potential targets for assassination — including himself and Litvinenko.

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

But in comments reported by the Moscow Times newspaper yesterday, an opposition lawmaker in the Russian parliament’s lower house noted that a law passed earlier this year allows Russian agents to operate abroad. “After we had adopted the bill that allows the deployment of special services abroad to fight terrorism, I would not rule out that our guys pulled this off,” Communist lawmaker Viktor Ilyukhin said, according to the newspaper.

Doctors had earlier given Litvinenko a 50 percent chance of survival. Nathwani said the patient was “seriously ill but stable” in the hospital’s intensive-care ward.

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 specialising in terrorism and organised crime in 1991, and was transferred to the FSB’s most secretive department on criminal organisations 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.