Traces of radiation have been found at roughly a dozen sites in Britain and five jets are now being investigated in connection with a probe into the poisoning of a former Russian spy, British officials say.

Home Secretary John Reid told Parliament Thursday that authorities are examining five planes for radiation contamination.

Two British Airways Boeing 767s grounded at London's Heathrow Airport have already tested positive, while a third was to be tested in Moscow.

A British Airways plane grounded at London's Heathrow airport.
(CBC)

Those jets, which operated between London and Moscow, show traces of polonium 210, the radioactive toxin that killed former Russian spy Alexander Litvinenko. A fierce critic of the Kremlin, he died last week in a London hospital after falling ill in early November.

The searches of the planes were prompted by a statement by Litvinenko that a group of Russian contacts who met with him in early November had travelled to London from Moscow.

The fourth aircraft, a Boeing 737 operated by the Russian airline Transaero, arrived at Heathrow airport early Thursday morning, said Reid.

"Passenger details will be collected and the Health Protection Agency will be contacting any individuals if any matters of concern are found," he said.

A spokesperson for Transaero later said no radioactivity was discovered on board, but British officials haven't yet commented.

Reid didn't offer any details about the fifth plane, which he said was of interest to authorities. It is also believed to be a Russian carrier.

"There may be other airplanes of which we don't at this stage know," Reid said.

Airline contacting passengers

Reid said authorities are monitoring about 24 sites in Britain and that radiation contamination has been confirmed at "around 12 of these venues." It's not clear whether the contamination is polonium 210.

Eighteen people who may have been exposed to radiation contamination have been referred to a special clinic for further testing.

British Airways is attempting to contact 33,000 passengers and 3,000 staff and ground crew who may have had contact with the jets. In the past three weeks, those planes carried out 220 flights to destinations across Europe, including Barcelona, Frankfurt and Athens.

The airline has published the flights affected on its website and told customers on these flights to contact a special help-line set up by the Health Ministry.

Public risk is low, officials say

The public health risk is low, said Dr. Michael Clark, a spokesman for the United Kingdom's Health Protection Agency.

"Say there was some residue left on the arm of a chair and a child sucked on it," he told CBC News on Thursday. "The doses are below dose limits for members of the public, so there isn't a significant risk."

The Health Protection Agency is monitoring the public health risk in this investigation.

In the room where Litvinenko stayed, the health agency checked the halls and adjacent rooms for radiation. The agency is also testing the urine of people who either came in contact with Litvinenko or the places where radiation has been detected.

Ariel Fenster, a chemistry professor at Montreal's McGill University, said polonium 210 is only dangerous when ingested. He said it requires sophistication to obtain and handle it.

"It's still a puzzle how polonium 210 got into so many planes," he told CBC News on Thursday.

Also Thursday, doctors in Moscow said former Russian prime minister Yegor Gaidar may have been poisoned during a conference last week in Ireland, the Associated Press reported. He is in stable condition in a Moscow hospital.

Implicated Putin in his death

Litvinenko became ill in early November and told police he believed he was poisoned on Nov. 1 while dining at a London sushi bar, where he met a contact during his investigation into the slaying of Russian journalist Anna Politkovskaya.

Alexander Litvinenko at the University College Hospital in central London on Nov. 20.
(Natasja Weitsz/Getty Images)

Like Litvinenko, the murdered reporter had been critical of Russian President Vladimir Putin's government.

Within three weeks, Litvinenko's hair fell out, his throat became swollen, and his immune and nervous systems suffered severe damage.

British officials later said they found traces of polonium 210 — usually manufactured in specialized nuclear facilities — in his urine.

He died accusing Putin of orchestrating his murder, but London police are still investigating it as a "suspicious death" and have not ruled out the possibility Litvinenko may have poisoned himself.

His autopsy was to be conducted on Friday.

With files from the Associated Press