British Airways has discovered traces of radiation on two of its three aircraft that have flown to Moscow since the death of Alexander Litvineko.

Two Boeing 767s tested at Heathrow showed “very low traces of a radioactive substance”. A third aircraft, which has been grounded in Moscow, is due to be tested.

The Health Protection Agency would not say if the substance found was polonium-210, the radioactive isotope that poisoned the former KGB spy.

The aircraft were immediately withdrawn from service, but the airline and toxicologists said that there was little risk to the public.

Willie Walsh, the chief executive of British Airways, said last night that up to 33,000 passengers had flown on the three aircraft. In all, at least 221 flights have potentially been affected.

It is understood that Scotland Yard is investigating flights from Moscow to Heathrow from October 25. Since this date the three planes have flown to destinations around Europe.

Mr Walsh said that a further 3,000 BA and airport staff had been involved with the three aircraft. He added: “We want to reassure our customers that the advice we have been given is that the risk to the public is very low.”

Police have been trying to establish where the polonium-210 came from and have studied passenger lists on flights to and from Moscow.

Two of the Russians Mr Litvinenko met on November 1, the day that he fell ill, had only recently arrived from Moscow. Both men have denied being involved in any plot and tests on them have shown no contamination.

One possibility being studied was that the poisoner, or his accomplices, may have been exposed to polonium. Police believe that the poisoner could have arrived in London up to a week before Mr Litvinenko was taken ill.

Andrei Lugovoi, one of the two Russians Mr Litvinenko met for tea in London, told the Russian paper Kommersant last night that he had flown to Moscow on November 3 on one of the contaminated aircraft.

It is understood that police have identified a man they wish to question who was staying at a London hotel, having flown from Moscow at the time that Mr Litvinenko, 43, fell ill. Mr Lugovoi has denied any involvement in Mr Litvinenko’s poisoning.

None of the other people Mr Litvinenko came into contact with on that day has tested positive for any exposure.

An airline spokesman said that the aircraft were being examined because “individuals involved in the Litvinenko case” had travelled on them.

John Reid, the Home Secretary, is to make a statement to MPs on the latest developments in the affair later today. He chaired a meeting of Cobra, the Cabinet’s emergency committee, yesterday to assess the risk to the public.

The priority for police is to establish Mr Litvinenko’s route and use CCTV footage to try to discover when and how he was given the poison before they work their way through theories about who wanted the 43-year-old dissident dead. So far they have not accounted for all the time Mr Litvinenko was in London.

Mario Scaramella, an Italian academic met the former Kremlin agent at the Itsu sushi bar. He insisted yesterday that police did not regard him as a suspect.

After a day of questioning in London by police, Mr Scaramella said: “I’m not under investigation by any British authority. I am co-operating with them [the police].”

Mr Scaramella, who worked on an Italian parliamentary investigation into KGB dirty tricks in Rome, said that tests he had taken showed that he had not been contaminated by polonium-210.Toxicologists again checked rooms and public areas in the five-star Sheraton Park Lane hotel in Mayfair yesterday, but again found no trace of radioactive contamination.

A doctor from the Health Protection Agency met the tenants of offices at 58 Grosvenor Street to assure them that there were no risks to their health. A post-mortem examination on Mr Litvinenko is to be carried out tomorrow.

The flights

BA875 Moscow-Heathrow on October 25

BA872 Heathrow-Moscow on October 28

BA873 Moscow-Heathrow on October 31

BA874 Heathrow-Moscow on November 3

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