Russia's foreign intelligence service issued a sharply worded denial on Wednesday of involvement in the poisoning of a fierce Kremlin critic and former spy who fell seriously ill in Britain, the Interfax news agency reported.

Alexander Litvinenko is in a London hospital under armed guard after what his associates claim was an assassination attempt by the Russian government. Doctors say he clearly was poisoned, but the substance used has not been identified.

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"Litvinenko is not the kind of person for whose sake we would spoil bilateral relations," Interfax quoted Sergei Ivanov, a spokesman for the Foreign Intelligence Service, as saying. "It is absolutely not in our interests to be engaged in such activity."

The service, whose Russian abbreviation is SVR, declined comment to The Associated Press.

The SVR is one of the successor agencies to the Soviet KGB.

Ivanov's comments echoed previous Russian denials of involvement in the poisoning.

But in comments reported by the Moscow Times newspaper on Tuesday, 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.

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 and a year later spent nine months in jail on charges of abuse of office, of which he was later acquitted.

He fled to Britain in November 2000, and three years later wrote a book accusing the FSB of staging apartment-house bombings that authorities blamed on Chechen separatists.

Litvinenko became ill shortly after he met with an Italian security expert, who said he had shown Litvinenko e-mails from a confidential source identifying the killers of journalist Anna Politkovskaya, a strong critic of the Kremlin who was gunned down in Moscow in October. Litvinenko has accused Russian President Vladimir Putin in the killing.

Doctors initially said that Litvinenko, 43, appeared to have ingested the toxic metal thallium, but the hospital where he is in intensive care said Tuesday that says that tests show thallium poisoning was unlikely. He has suffered damage to the immune system and to vital organs, as well as hair loss.

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.

Radioactive thallium is commonly used in hospitals, injected into the bloodstream to act as a tracer during heart scans.