All seven bar staff working at the Pine Bar in the Millennium Hotel that night have tested positive for polonium-210, the radioactive isotope that killed Litvinenko. Health authorities are trying urgently to contact the 250 customers using the busy bar on November 1.

The Pine Bar is a popular haunt for businessmen and foreign guests at the five-star hotel in Grosvenor Square. Many of those overseas travellers have returned home after being told they were at no risk.

Health experts said they were surprised to find that the levels of radiation found in the seven bar staff approached that found in Litvinenko’s wife, Marina. Professor Pat Troop, of the Health Protection Agency, said there was no short-term danger to the bar staff but conceded that there was a “very small” long-term risk of cancer.

Michael Clark, of the agency’s radiation protection division, said last night that it was possible that Litvinenko was poisoned by a contaminated cigarette or drink.

A minute quantity of polonium-210 placed in Litvinenko’s glass would explain how he ingested the radioactive poison that led to his agonising death three weeks later.

The vapour that evaporated from the drink would have been inhaled by anyone in the area, with a greater concentration for his Russian companions and staff, who would have been in the bar much longer.

Investigators believe the poison cocktail was likely to have been manufactured in a guest room at the hotel, a short walk away from the US Embassy. Significant traces of polonium-210 were found in a fourth-floor room, which was occupied by a visiting Russian.Police believe that the killer may have stalked Litvinenko in London that day and had first tried to poison the ex-KGB colonel in a sushi bar. That failed but the poisoner left ample traces of the deadly radioactive isotope in the Piccadilly restaurant. Traces were also found on an Italian academic, Mario Scaramella, who was in the Itsu sushi bar. Toxicologists found polonium-210 in every place that Litvinenko visited after his drink at the hotel. It was not until he arrived home two hours later that he was violently ill.

Litvinenko told police he felt no ill-effects as he went on to the hotel to discuss a business deal with Andrei Lugovoy, a former KGB colonel, and his partner, Dmitry Kovtun.

Reports last night that Mr Kovtun had suddenly collapsed in a coma in Moscow with radiation poisoning were denied by his lawyer, hours after the Interfax news agency claimed that Mr Kovtun was in critical condition in a clinic. Mr Kovtun had spent the past two days at the clinic talking to Russian prosecutors and British detectives.

He joked that on the night “the only poison we [he and his business partner Mr Lugovoy] gave Litvinenko was alcohol”.

British police were again told yesterday that they will have to wait to see Mr Lugovoy. He met Litvinenko four times in the fortnight before he was poisoned. He has denied having any part in his death.

The Russian authorities announced last night that they were opening their own investigation, claiming that the Russian businessmen were also targets of the killer. The move is likely to hijack the inquiry from the Scotland Yard team in Moscow. Marina Gridneva, a spokeswoman, for the Prosecutor-General said that Moscow could decide to send a team of prosecutors to Britain. She said that the investigation was intended to “deepen co-operation” with Scotland Yard.

• Traces of polonium-210 has been found at Parkes Hotel, Mayfair, it was confirmed last night. It means that radiation has been found at all three hotels where Mr Lugovoy had stayed since flying to London on October 16. The Parkes was the first he stayed at.

  The radioactive isotope has also been found at Risc Management, a security firm in Cavendish Place, visited by Litvinenko with Mr Lugovoy and Mr Kovtun on October 17.

Times Recommends

• Postmen: We bind the nation together
• Tewkesbury: when the memories flood back
• Cast Offs actress: I’m happy that my face fits