British detectives have questioned a Russian businessman who entertained Alexander Litvinenko in a London hotel on the day the former spy fell ill.

At the same time Scotland Yard confirmed that the investigation into Litvinenko’s death by radiation poisoning had become a murder inquiry.

Police have been following the trail across London left by Dimitri Kovtun and his close friend, Andrei Lugovoy. It includes a number of locations where polonium-210 has been found.

Mr Kovtun was interviewed by Russian prosecutors yesterday in the presence of the British detectives, who arrived in Moscow on Monday. He and and Mr Lugovoy were in the same Moscow clinic last night being tested for contamination by polonium-210.

Diplomats in the Russian capital also confirmed that traces of radiation had been discovered in the British Embassy, which the businessmen visited shortly after news of Litvinenko’s poisoning became public. The two men gave written statements to embassy officials and expressed their willingness to co-operate in any British inquiry.

The lawyer representing the two businessmen, Andrei Romashov, emphasised that they were being treated “as witnesses”. He disclosed that Mr Kovtun had also been interviewed briefly on Tuesday about his two trips to London.

A third Russian businessman, Vyacheslav Sokolenko, who also flew to London to watch a football match on November 1, is on the list of men British detectives want to meet.

All three men met Litvinenko at the Millennium Hotel in Grosvenor Square, London, and were among the last to see him before he succumbed to the effects of the radiation poison. They have all strenuously denied any part in his death.

Mr Lugovoy, a former KGB officer whose company is worth £50 million, has said that he believes he has been framed and is worried about how seriously he has been contaminated by polonium-210. There were reports last week from Moscow that he had been tested and showed no signs of exposure.

He denies he is trying to avoid a meeting with the detectives and says that he is fed up with allegations that he was involved in the poison plot.

Detectives working on the London end of the murder inquiry are concentrating on identifying where Mr Lugovoy went and whom he met during three trips he made in the fortnight before his former colleague was fatally poisoned.

What police have found startling is that at eight locations on the Lugovoy trail traces of polonium-210 have already been found; experts are still testing other sites.

Mr Lugovoy visited five of the sites with Litvinenko, including the Itsu sushi bar where friends of the former KGB colonel suspect he was poisoned. Mr Lugovoy says that he went there on October 16 at Litvinenko’s invitation. He told The Times: “It was one of his favourites in London. He said it was a good place to talk.”

But the poison victim was never at the Arsenal football ground, where Mr Lugovoy and a party of wealthy Russians went to watch a match on November 1. Nor was he on either of the two British Airways Boeing 767s on which Mr Lugovoy flew from Moscow and on which radioactive material was discovered.

Police are surprised that polonium-210 was found in guest rooms at both the Millennium Hotel and at the Sheraton Park Lane, when Mr Lugovoy maintains that he met Litvinenko only in the public bars and the foyers of both hotels. Mr Lugovoy’s version of events is that whoever poisoned his former colleague also contaminated him to implicate him in the plot and draw police away from the real culprits.

He and Mr Kovtun have been friends since their teenage days, when they went to the same military academy. The third man on their football trip to London, Mr Sokolenko, was at the same army college.

Mr Kovtun says that together they built up the Pershin company and that they met Litvinenko to discuss a business proposal.

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