British Airways is trying to contact 33,000 passengers after radioactive traces were found on two of its planes.
Mr Litvinenko was a fierce critic of Russian President Vladimir Putin
A third plane, currently in Moscow, is to be flown back to the UK for tests.
The low grade radiation was found by scientists investigating the death of ex-spy Alexander Litvinenko. An inquest has been opened and adjourned.
BA has named 221 affected flights on its website and passengers are urged to contact NHS Direct or their own doctor. The health risk is thought to be low.
A spokeswoman for BA said the airline had also been "proactively calling passengers" and hoped to have contacted the majority by the end of Thursday.
It has so far taken calls from 2,500 customers on a dedicated helpline.
An estimated 3,000 staff would also need to be checked, BA said.
The alert involves 221 flights made by the three short-haul 767s in Europe between 25 October and 29 November, almost a quarter of which were between Moscow and London.
Mr Litvinenko, an ex-KGB agent and a fierce critic of Russian president Vladimir Putin, died last week of radiation poisoning.
Traces of radioactive polonium-210 were discovered in his body and more traces of the substance were found at venues he visited in the capital on 1 November.
All flight numbers published on the BA website
Scotland Yard has not said why it became interested in the planes, which were used on flights to Moscow and other European destinations over a five-week period.
But detectives are known to be tracing the movements of those who associated with Mr Litvinenko.
One of the two Russians who met Mr Litvinenko on the day he fell ill has told Russian newspaper Kommersant that he travelled on one of the planes involved on 3 November.
Former KGB bodyguard Andrei Lugovoi told the paper he had had nothing to do with the poisoning.
He met the ex-spy after his meeting at a West End sushi restaurant with an Italian security consultant, at which Mr Litvinenko was already showing signs of radiation contamination.
An autopsy will be carried out on Friday.
After the adjournment, Alex Goldfarb, a friend of Mr Litvinenko, said police investigations into flights between Moscow and London reinforced the theory that the radioactive substance blamed for his death had come from Russia.
Police were "looking obviously for the ways of delivery of this material to London", he told journalists outside the court.
British Airways chief executive Willie Walsh told the BBC that the aircraft affected were all of the same type, and were being carefully examined.
Contact with carrier's sweat or urine could lead to exposure
But polonium-210 must be ingested to cause damage
Radiation has very short range and cannot pass through skin
Washing eliminates traces
"Three specific aircraft were initially identified - three 767s," he said. "Two of those aircraft have been tested, and very low levels of radioactive traces have been discovered on the aircraft."
Mr Walsh said that the aircraft had made a large number of flights since they were contaminated, carrying many thousands of passengers, and the company was trying to alert them all.
The BBC's Gordon Correra says the police focus on flights as far back as October 25 could be significant if investigators discover that radioactive traces were left on board before Mr Litvinenko fell ill.
This could indicate they were left by people bringing the material into the country rather than by those who had been in contact with the ex-spy.
Chief executive of Britain's Health Protection Agency Professor Pat Troop said that if the source of the radiation was the same as that which killed Mr Litvinenko - polonium-210 - the risk of serious contamination to passengers was small.
"What we have heard is that it's either traces or very low levels and what we have learnt so far in our investigation... is that where we have got these areas of low level radiation it doesn't seem to pose a significant health threat," he said.
Home Secretary John Reid is expected to make a statement to Parliament concerning the investigation today.
British Airways has set up a special helpline for customers in the UK on 0845 6040171 or +44 191 211 3690 for international calls.
Passengers who travelled on those flights and want further advice are advised to telephone NHS Direct on 0845 4647.