Three of BA’s Boeing 767s were tested, but the airline said today that only two showed “very low traces of a radioactive substance”.

The Health Protective Agency would not say if it 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 The aircraft were immediately withdrawn from service, but the airline and toxicologists said there was little risk to the public.

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.

One possibility being studied was that the poisoner — or his accomplices — may have been exposed to polonium themselves. It is believed the poisoner could have arrived in London up to a week before Mr Litvinenko was taken ill.

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 the November 1, the day 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.

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

An estimated 33,000 passengers have flown on the affected aircraft over the past four weeks.

BA said: “The initial result of the forensic tests, which was confirmed late this afternoon, has shown very low traces of a radioactive substance on board two of the three aircraft.”

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 tomorrow. He chaired a meeting of Cobra, the Cabinet’s emergency committee,today to assess the risk to the public. The priority for police is to establish Mr Litvinenko’s precise route and use CCTV footage to try to discover when and how he was given the poison before they work their way through a welter of 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 that day.

Mario Scaramella, an Italian academic met the former Kremlin agent — who had recently become a British citizen — at the Itsu sushi bar. He insisted that police did not regard him as a suspect.

After a day of questioning in London by police, Mr Scaramella said: “Let me take the opportunity to say that I’m not under investigation by any British authority. I am co-operating with them .”

He was reacting to claims from some friends of Mr Litvinenko, who had implicated the Italian security expert in the poison plot.

Mr Scaramella, who worked on an Italian parliamentary investigation into KGB dirty tricks in Rome, said that tests he had taken in London showed that he had not been contaminated by polonium 210.

Following their poison trail through the capital, toxicologists again checked a number of rooms and public areas in the five-star Sheraton Park Lane hotel in Mayfair today, but again found no trace of radioactive contamination.

Staff at the hotel told The Times that officers were searching a particular guest’s room, but said that the police would not tell them how this person was linked to Mr Litvinenko. It is not known if he visited the hotel during his tour around the West End on November 1.

Police also examined a suite of offices in a mansion block in Grosvenor Street, but again there was no trace of radiation. A doctor from the Health Protection Agency met tenants of offices at 58 Grosvenor Street to assure them that there were no risks to health.

A post-mortem examination on Mr Litvinenko is to be carried out on Friday.