Traces of a radioactive substance have been found on two British Airways aircraft at Heathrow Airport in London.
A total of three Boeing 767s have been taken out of service and are being examined as part of the investigation into the death of the former Russian spy Alexander Litvinenko.
Two planes at Heathrow Airport in London were withdrawn after tests showed low levels of radiation, while a third aircraft remains on the ground in Moscow. A team of British experts will travel to Russia to carry out tests on that plane.
In a statement, British Airways said that the initial results of the forensic tests have shown very low traces of a radioactive substance on board two of the three aircraft.
The airline said it understands that the risk to public health is low.
Nevertheless, British Airways is contacting all passengers who flew on the affected planes between 25 October and 28 November.
During those dates the aircraft involved flew between Heathrow Airport and several destinations including Moscow, Barcelona, Dusseldorf, Athens, Larnaca, Vienna and Stockholm.
In all BA is looking at 221 flights involving around 33,000 passengers.
The full list of flights involved can be found here
The airline says it will be contacting all passengers but has advised anyone who is concerned to contact their doctor.
Former Russian spy Alexander Litvinenko died last week from radiation poisoning after traces of Polonium 210 were found in his body.
On 1 November, the day he first fell ill, he had met two Russians in a hotel in central London.
It is known that at least one of those men flew back to Moscow on 3 November.
Scotland Yard will now be looking at the possibility that traces of the radioactive substance could have got onto the planes through someone who had been in contact with Mr Litvinenko or could have come from someone bringing the substance into Britain.
