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 involvement of a former FSB officer made it easier to lure Mr Litvinenko to meetings at various locations and to distance its bosses in the Kremlin from being directly implicated in the plot.

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.”

Nine Scotland Yard detectives are in Moscow, and they 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”.

British ministers insist that diplomatic sensitivities will not be allowed to obstruct the scope of the Yard investigation.

John Reid, the Home Secretary, who was also in Brussels briefing his European counterparts on the Litvinenko affair, said: “The police will follow the evidence wherever it goes.”

The main figure that the British counter-terror team want to question is Andrei Lugovoy, a former FSB agent.

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