The death of the former Russian agent Alexander Litvinenko is now being treated as murder, Scotland Yard said last night.
The announcement is likely to increase the diplomatic tension surrounding efforts to catch those responsible for poisoning the exiled former spy.
Officers from the Yard's anti-terrorist branch continued efforts in Moscow to interview potential witnesses to the attack on Mr Litvinenko, who was targeted with a large quantity of polonium 210 in London five weeks ago. The dose is said to have been worth millions of pounds.
Although the change in the status of the case will have little material effect on how the inquiry is conducted, it was made against a background of increasingly tense relations between the British and Russian authorities.
Officers have followed a trail of evidence to Moscow, including traces of polonium 210 found on two British Airways aircraft used on flights to the city.
On Tuesday, the Russian prosecutor general, Yuri Chaika, announced that the British detectives would not be allowed to directly question anyone and no Russian citizens would be extradited to Britain to face trial.
In a statement last night, the Yard said: "Detectives investigating the death of Alexander Litvinenko have reached the stage where it is felt appropriate to treat it as an allegation of murder."
The Yard, which had initially said it was treating Mr Litvinenko's death as "suspicious", said it was pursuing "many lines of inquiry" in Britain and Russia. The statement added: "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."
But the Foreign Office confirmed that small traces of radiation had been found in a room in the British embassy in Moscow. It is understood the traces were found in a room used for a meeting between embassy staff and Andrei Lugovoi, a businessman and ex-KGB bodyguard who met Mr Litvinenko at a London hotel on 1 November, the day he fell ill.
The Yard officers had their meeting with Mr Lugovoi delayed for a second time yesterday while the Russian undergoes tests for radiation poisoning.
Representatives of Mr Lugovoi, who has suggested he is being framed as a suspect, said they had not been given a date or time for the interview by Russian authorities.
Detectives are understood to have held two meetings with a business associate of Mr Lugovoi, Dmitry Kovtun, who also met Mr Litvinenko on 1 November.
In London, preparations were being made for Mr Litvinenko's funeral tomorrow amid reports that he had converted to Islam in the days before his death.
The former spy's father, Walter, told a Moscow radio station: "He told me about his decision two days before he died. He said, 'Papa, I have to talk to you about something serious. I've become a Muslim.'"
Tests for polonium 210 on staff at the Itsu restaurant where Mr Litvinenko ate before falling ill were negative, the chain said yesterday.