British Home Secretary John Reid has told the House of Commons that traces of radiation have been found at 12 out of 24 locations investigated by experts after the death of Alexander Litvinenko.
Mr Reid said that early results of tests on two British Airways planes at Heathrow showed low levels of a radioactive substance on board both aircraft.
The aircraft were tested after traces of a radioactive substance were found on them by scientists involved in the investigation into the death of Mr Litvinenko, a Russian former spy.
The planes will undergo further examinations today and another 767 grounded in Moscow will be given a preliminary check but will not be returned to London until the position becomes clearer.
The three planes had flown between London and Moscow, as well as several other European destinations, between 25 October and 28 November.
A fourth aircraft - a Boeing 737 leased by Transaero - arrived at Heathrow this morning and is being monitored by scientists.
BA is attempting to contact all passengers who flew on their flights.
Mr Reid said that following the radiation discovery 1,700 calls had been made to NHS Direct, 69 people have been referred to the Health Protection Agency, of whom 18 have been referred to clinics.
Mr Litvinenko, who was a former Russian intelligence officer and a strong critic of President Vladimir Putin, died of radiation poisoning last week.
A coroner in London today formally opened and adjourned an inquest into the death of Mr Litvinenko as the police investigation continues.
The Russian government has denied it was involved in his death.
