British intelligence services, who has continued their inquest in death of Alexander Litvinenko on Russian soil, are convinced the poisoning of former Russian spy was authorised by FSB, Russia's secret service.

Security sources in London, as quoted by Times, say the FSB, which stands for Federal Security Bureau, orchestrated a "highly sophisticated plot" to murder Litvinenko, its former lieutenant coloner who exiled to the UK six years ago.

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

British intelligence officials say only high-ranking state security officials would have been able to obtain a sufficient quantity of polonium-210, the radioactive substance used to poison Litvinenko just weeks after he was given British citizenship following his defection from Russia.

A senior police source said the method used to kill the 43-year-old Russian 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."

British spy agencies MI5 and MI6 are working closely with Scotland Yard on the investigation. British intelligence officials believe a sizeable team was sent from Moscow to smuggle radioactive polonium-210 into Britain and to shadow Litvinenko.

Nine Scotland Yard detectives have landed yesterday in Moscow. They plan to question a number of well-connected businessmen, despite a warning yesterday from Russian Foreign Minister Sergey Lavrov that speculation over the fatal poisoning would further strain relations between Russia and the UK.

"It's unacceptable that a campaign should be whipped up with the participation of officials," Lavrov said during a visit to Brussels.

"This is of course harming our relations."

Lavrov said he had spoken with British Foreign Secretary Margaret Beckett about the case and the "necessity to avoid any kind of politicisation of this matter, this tragedy".

Russian President Vladimir Putin has also accused Britain of mounting a propaganda campaign over the case.

Two hotels in London in which Alexander Litvinenko met his Russian colleague, Andrey Lugovoy, had traces of polonium-210, as did a British Airways plane he travelled on. At least one more person who met with Litvinenko the day he fell ill with radiation poisoning, Italian professor Mario Scaramella, has been tested positive for radioactive contamination.

NHS Direct had received 3,025 calls from people who suspected they could've been in locations visited by Litvinenko the days he fell sick. Up to 179 people will undergo medical tests, and 27 were referred as a precaution to a specialist outpatient assessment clinic for possible radiological exposure assessment, British Health Protection Agency said.

Not suprisingly, British ministers insist diplomatic sensitivities will not be allowed to obstruct the scope of the investigation.

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