Traces of radiation have been discovered at the British embassy in Moscow, officials said tonight.

The find by Scotland Yard experts coincided with the news that the death of Alexander Litvinenko is now being treated as a murder and is likely to cast fresh suspicions on two Russian businessmen who met the former spy on the day that he fell ill in London.

Andrei Lugovoy, a former KGB officer, and his business partner Dmitri Kovtun visited the embassy shortly after news of Mr Litvinenko’s illness from radiation poisoning became public. Both men gave written statements to embassy officials and expressed their willingness to cooperate with the police inquiry.

They are currently in a Moscow hospital, apparently receiving treatment for exposure to radiation.

An embassy spokesman refused to say last night that the source of the radiation had been identified as polonium-210, the isotope that killed Mr Litvinenko. He said that the experts had found "small traces of radiation at very low levels", but had established that there was no risk to health and the embassy continued to work normally.

Mr Kovtun was interviewed last night by Russian prosecutors in the presence of British detectives, who arrived in Moscow on Monday in a search for clues about the poisoning of Mr Litvinenko. The lawyer representing the two men, Andrei Romashov, said Mr Kovtun had also been interviewed on Tuesday. He said that investigators were treating both men "as witnesses".

Mr Lugovoy said that he was ready to meet the detectives and is expected to meet them either tomorrow or on Friday. A third businessman, Vyacheslav Sokolenko, is also expected to be interviewed. All three men met Litvinenko at London’s Millennium Hotel on November 1 and were among the last to see him before he was admitted to hospital. He died on November 23. They have all strenuously denied any part in his death.

Mr Lugovoy, who runs a private security company, has suggested that he is the victim of a plot to frame him.

The British detectives are interested in Mr Lugovoy because aircraft on which he travelled to and from Moscow were found to be contaminated with radiation, along with rooms in two London hotels where he stayed.

Mr Lugovoy has said that he met Litvinenko to discuss a business proposal and had been in London with his wife and three children on November 1 to watch the Champions League match between Arsenal and CSKA Moscow. The Health Protection Agency confirmed today that it had found minute traces of polonium-210 at two locations in Arsenal’s stadium.

This evening, the Metropolitan Police said that it was "now appropriate" to treat the death of Litvinenko as a murder.

"Detectives in this case are keeping an open mind and methodically following the evidence. It is important to stress that we have reached no conclusions as to the means employed, the motive or the identity of those who might be responsible for Mr Litvinenko’s death.

"The inquiry is still in its early stages and we are not prepared to comment on speculative reports in the media. We will provide further information as and when appropriate."

As Scotland Yard suggested that its inquiry becoming more serious, Russian newspapers chose to give prominent coverage to the strict limits imposed on British detectives in Moscow by the Prosecutor General, Yuri Chaika.

Most concluded that the British team had been rendered toothless by Mr Chaika’s declaration that Russian officials would conduct any interviews and that there was no question of anybody being extradited to the UK.

The Vremya Novostei newspaper headlined its front-page report with Mr Chaika’s comment that "We are doing the interrogating and they (Scotland Yard) are present". It said that the detectives’ visit was "hardly likely to help them really get closer to solving the secret of the death of Alexander Litvinenko".

Gazeta said that Scotland Yard had been forced to take a "time out" just a day after their arrival.

After Mr Chaika’s abrasive press conference on Tuesday, the British team spent part of yesterday in discussions with the Prosecutor General’s office. Russian officials told Interfax news that talks had centred on "details of the witnesses’ questioning".

The Prosecutor General’s Office said later in a statement: "Despite the fact that the UK’s request for legal assistance has some departures from international standards (currently these shortcomings are being addressed), Russian and British officers have begun the investigations."

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