A handout image shows former Russian spy Alexander Litvinenko at the University College Hospital in central London. Litvinenko was moved to intensive care in a London hospital, doctors said, as Moscow denied being behind his suspected poisoning.[AFP]

London - Officials found traces of radiation on two British Airways jets, and the airline appealed Wednesday to tens of thousands of passengers who flew to Moscow or other cities to come forward -- the latest twist in the inquiry into the poisoning death of a former Russian spy.

The airline said the "risk to public health is low," adding that it was in the process of contacting tens of thousands of passengers who flew on the jets.

Two planes at London's Heathrow Airport tested positive for traces of radiation and a third jet was taken out of service in Moscow awaiting examination, British Airways said.

Natalia Remnyova, administrator at Domodedovo Airport, the Moscow airport used by British Airways, said she knew nothing of a plane grounded there. Kremlin spokesman Dmitry Peskov said he could not comment because he had no information about the matter.

The airline said it was contacted by the British government Tuesday night and told to ground the jets and to let investigators looking into the death of former Russian spy Alexander Litvinenko test them for radiation.

High doses of polonium-210 -- a rare radioactive element usually made in specialized nuclear facilities -- were found in Litvinenko's body, and traces of radiation have been found at sites in London connected with the inquiry into his death.

It was not immediately clear how radioactive traces got on the British Airways planes. Authorities refused to specify whether the substance detected on the jets was polonium-210.

All three planes had been on the London-Moscow route, British Airways said. In the last three weeks, the planes had also traveled to routes across Europe including Barcelona, Frankfurt and Athens. Around 33,000 passengers had traveled on 221 flights on those planes, said Kate Gay, an airline spokeswoman.

"The airline is in the process of making contact with customers who have traveled on flights operated by these aircraft, which operate within Europe," British Airways said in a statement.

"British Airways understands that from advice it has been given that the risk to public health is low," the airline statement said.

The airline has published the flights affected on its Web site, and advised customers who took the flights to contact a special help-line set up by the British Health Ministry.

Litvinenko, a former colonel with Russia's Federal Security Service -- the successor agency to the KGB -- had been a fierce critic of Russian President Vladimir Putin before his death from radiation poisoning on Nov. 23. From his deathbed, he blamed Putin for his poisoning. Putin has strongly denied the charge.

Alexander Litvinenko, former KGB spy and author of the book 'Blowing Up Russia: Terror From Within' photographed at his home in London in this Friday, May 10, 2002 file photo. [AP]

Britain's Home Secretary John Reid, who chaired a meeting of COBRA, the government's emergency committee, said tests on the planes were part of a wider scientific investigation into sites that could be linked to Litvinenko's death.

Meanwhile, Italian security expert Mario Scaramella, who was one of the last people to meet with Litvinenko before the former spy fell ill, said tests cleared him of radioactive contamination.

Scaramella came from Rome and met Litvinenko at a sushi bar in London on Nov. 1 -- the day the former intelligence agent first reported the symptoms.

"I am fine," Scaramella told The Associated Press by telephone. "I am not contaminated and have not contaminated anybody else."

Scaramella returned to London to undergo tests and talk with the police Tuesday. He said he is in security protection and refused to say where he was.

More than three dozen staff at the two hospitals that treated Litvinenko will be tested for radioactive contamination, Britain's Health Protection Agency said.

The agency said 106 staff at Barnet General Hospital and University College Hospital had been assessed for possible exposure, and 49 would have their urine tested.

The mysterious death has clouded Anglo-Russian relations. Prime Minister Tony Blair said police were determined to find out who was responsible for Litvinenko's death.

"The police investigation will proceed, and I think people should know that there is no diplomatic or political barrier in the way of that investigation," Blair said in Denmark. "It is obviously a very, very serious matter indeed. We are determined to find out what happened and who is responsible."

Media reports in Britain and Russia on Wednesday said that Litvinenko had been engaged in smuggling nuclear substances out of Russia.

The Independent newspaper reported that Litvinenko told Scaramella on the day he fell ill that he had organized the smuggling of nuclear material for his former employers at Russia's Federal Security Service, or FSB. The newspaper reported that Litvinenko said he had smuggled radioactive material to Zurich in 2000.

But Scaramella told the AP that he had been misquoted by the newspaper.

"He (Litvinenko) wanted to see me because he knew about smuggling of nuclear material, but as far as I know he was never involved in nuclear smuggling," he said.

London police say they are investigating the case as a "suspicious death" rather than murder, although they have devoted a large anti-terrorist force to the investigation.

Scaramella said he had been cleared of any involvement in the 43-year-old former spy's death.

"Let me take the opportunity to say that I'm not under investigation by any British authority," he said. "I am cooperating with them (the police)." Police declined to say whom they had spoken to.

Scaramella said he showed Litvinenko e-mails from a confidential source identifying the possible killers of Russian investigative journalist Anna Politkovskaya and listing other potential targets for assassination -- including himself and Litvinenko.

Following Litvinenko's death, more than 1,100 people called a health hot line over concerns they might be at risk from polonium poisoning, which is deadly in tiny amounts if ingested or inhaled. Sixty-eight have been referred to health authorities, the Health Protection Agency said -- including the 49 hospital staff.

Eight have been referred to a special clinic as a precaution. The tests should take about a week.

Traces of radiation have been found at six sites visited by Litvinenko.

A coroner will perform an autopsy on Litvinenko on Friday, "subject to appropriate precautions," said the local authority responsible, Camden Council. Doctors had sought expert advice on whether Litvinenko's radioactive body posed a threat to those performing the post-mortem.

A coroner's inquest will be opened Thursday and then adjourned until the police investigation is complete, the council said.