Ailing former Russian spy Alexander Litvinenko's condition has deteriorated and doctors remain puzzled by his critical illness, which probably was not caused by the toxic metal thallium or radiation, a doctor said Thursday.

A friend said Litvinenko was on life support.

Dr. Geoff Bellingan, director of critical care at University College Hospital, said Litvinenko's condition had shown "a dramatic deterioration" overnight. He did not comment on the report by Litvinenko's friend, Alex Goldfarb.

"We are now convinced that the cause of Mr. Litvinenko's condition was not a heavy metal such as thallium. Radiation poisoning is also unlikely," Bellingan said, reading a statement to reporters outside the hospital. "Despite extensive tests, we are still unclear as to the cause of his condition."

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Doctors originally suspected Litvinenko had been given thallium. They later said it is more likely he is suffering from the effects of another substance, possibly a radioactive one.

Friends and dissidents have alleged that an attack on Litvinenko was carried out at the behest of the Russian government. Litvinenko sought asylum in Britain in 2000, and has been a relentless critic of the Kremlin and the Russian security services.

Scotland Yard's anti-terrorist branch is investigating Litvinenko's illness.

On Wednesday, the Russian Foreign Intelligence Service, the SVR, issued a sharp denial that it was involved in any assassination attempt.

"Litvinenko is not the kind of person for whose sake we would spoil bilateral relations," SVR spokesman Sergei Ivanov said, according to the Interfax news agency. "It is absolutely not in our interests to be engaged in such activity."

The SVR, one of the successor agencies of the Soviet KGB, declined to comment to The Associated Press.

The British Broadcasting Corp., quoting an unidentified hospital source, reported that X-rays had shown that Litvinenko swallowed three objects of dense matter, which had lodged in his intestines.

The BBC said it was unclear whether the objects were related to Litvinenko's illness, and the hospital declined to comment.

Bellingan dismissed the reports.

"Suggestions that an X-ray identified three objects in his body are misleading," he said. "We are now convinced that shadowing on the x-ray was caused, as might be expected, by Prussian Blue, a nontoxic therapeutic agent which was administered as part of his treatment.

Goldfarb, the friend of the former KGB spy, told The Associated Press earlier in the day that his friend's heart has stopped working properly on its own.

"He went into a cardiac failure overnight," said Goldfarb, who alternately described Litvinenko as on a "ventilator," "artificial heart support" and "artificial resuscitation."

Goldfarb, who had joined Litvinenko's wife Marina and the former agent's father by his bedside, said the 43-year-old's condition had been deteriorating over the last few days.

"His heart has been generally weak of the past few weeks, he had low blood pressure yesterday, and his heart had stopped. He was in intensive care for that reason," Goldfarb said.

Litvinenko worked both for the KGB and for another successor, the Federal Security Service.

In 1998, he publicly accused his superiors of ordering him to kill tycoon Boris Berezovsky. He spent nine months in jail on charges of abuse of office but was later acquitted and moved to London.

Litvinenko said he had two meetings on the day he first felt ill. In the morning, he met at a London hotel with an unnamed Russian and Andrei Lugovoy, a former KGB colleague and bodyguard to one-time Russian Prime Minster Yegor Gaidar. Later, he dined with Italian security expert Mario Scaramella to discuss the October murder of another Kremlin critic — investigative journalist Anna Politkovskaya.

Litvinenko publicly blamed Moscow in Politkovskaya's death. The Kremlin has emphatically denied involvement in that killing.

Scaramella told reporters in Rome on Tuesday that he had traveled to meet Litvinenko to discuss an e-mail he received from a confidential source naming the killers of Politkovskaya, who was gunned down Oct. 7 at her Moscow apartment building. The e-mail also listed other potential targets, including Scaramella and Litvinenko.

Goldfarb said that he had a photocopy of the four-page e-mail and confirmed that it did read like the hit list described by Scaramella.

He refused to say who may have compiled the document, saying that it could jeopardize the police investigation into the poisoning.

After visiting the hospital on Thursday, Berezovsky told the AP that British police have yet to speak to him, but hoped they would be in contact over the next two days. The police declined to comment about whether they had the e-mail.

Goldfarb said Wednesday that there was nothing out of the ordinary in Litvinenko's meeting with Lugovoy, who also worked as bodyguard to Berezovsky, the most high-profile Russian exile in London.

Litvinenko has refused to implicate any of the people he met on the day he believes he was poisoned.

"He said there were two encounters held, but he is not accusing anybody. It could have happened then or it could have happened elsewhere," Goldfarb said.