Intelligence services in Britain are convinced that the poisoning of Alexander Litvinenko was authorised by the Russian Federal Security Service.

Security sources have told The Times that the FSB orchestrated a "highly sophisticated plot" and was likely to have used some of its former agents to carry out the operation on the streets of London.

"We know how the FSB operates abroad and, based on the circumstances behind the death of Mr Litvinenko, the FSB has to be the prime suspect," a source said yesterday.

The theory comes as the British police investigation moves to Russia where the British embassy in Moscow will today be tested for radiation.

The "precautionary tests" will be carried out in one room of the embassy by a team of experts who have travelled to Moscow with British police, the Foreign Office said.

Amid growing diplomatic tension over the continued furore, a spokesman stressed that the tests at the British embassy were just being undertaken as a precaution and the experts "did not expect to find anything".

Nine Scotland Yard detectives are in Moscow to question potential witnesses and those who met Litvinenko shortly before his death on November 23 after allegedly being poisoned with the radioactive isotope polonium-210.

A local Russian police force is likely to escort the British detectives during their trip, which could last several days or even weeks.

Intelligence officials say that only officials such as FSB agents would have been able to obtain sufficent amounts of polonium-210, the radioactive substance used to fatally poison Mr Litvinenko only weeks after he was given British citizenship.

MI5 and MI6 are working closely with Scotland Yard on the investigation. A senior police source told The Times yesterday that the method used to kill the 43-year-old dissident was intended to send a message to his friends and allies.

"It’s such a bad way to die, they must have known," the source said. "The sheer organisation involved could only have been managed by professionals adept at operating internationally."

The British officers are determined to question a number of well-connected businessmen, despite a warning yesterday from Sergei Lavrov, the Russian Foreign Minister, that speculation over the poisoning is straining relations between the two governments.

"It’s unacceptable that a campaign should be whipped up with the participation of officials. This is of course harming our relations," Mr Lavrov said during a visit to Brussels.

He said that he had spoken to Margaret Beckett, the Foreign Secretary, "about the necessity to avoid any kind of politicisation of this matter, this tragedy".