British Airways confirmed this evening that "very low traces" of radiation have been found on two of its aircraft by police probing the death of former Russian spy Alexander Litvinenko.
The airline is now attempting to track down 33,000 passengers who used more than 200 affected flights over the past month, as well as 3,000 staff.
British Airways said three B767 short haul aircraft were taken out of service for forensic examination.
Initial results showed "very low traces of a radioactive substance on board two of the three aircraft".
Mr Litvinenko, a former Russian spy, died last Thursday of radiation poisoning. Significant amounts of radioactive Polonium 210 were found in his body. Before he died, Mr Litvinenko accused Russian President Vladimir Putin of ordering his death.
The Kremlin denies any involvement but the case has fuelled tensions between London and Moscow. Police are treating his death as suspicious.
British Airways faces a huge challenge tracking down passengers who used these three planes on more than 200 flights, starting with a November 3rd London Heathrow to Moscow flight - shortly after Mr Litvinenko fell ill.
"We are looking at around 33,000 passengers on 221 flights over the past four weeks," a BA spokesman said. "We have set up a special helpline for customers," he added.
The British airline said the three planes would remain out of service until further notice. Two of the planes are at Heathrow Airport and the third is in Moscow and has not been checked yet.
BA said it had been advised "the risk to public health is low".
Home Secretary John Reid, who chaired a meeting today of the British government's Cobra emergency committee, confirmed tests had begun on two of the planes and arrangements were being made for the third to be tested.
He will make a statement to parliament on Thursday.
A post mortem will be carried out on Litvinenko on Friday under secure conditions to avoid possible radioactive contamination.
Mario Scaramella, an Italian KGB expert who met Mr Litvinenko the day he became ill, is under police protection and underwent medical tests.
He was not contaminated, his lawyer said today.
Mr Scaramella has denied media reports he is viewed as a suspect in the immensely complex case that spans Europe.