Russia's security agency said Friday it has launched an investigation into suspected British spying, based on information from Britain's chief suspect in the fatal poisoning of former agent Alexander Litvinenko.

The suspect, Andrei Lugovoi, claimed last month that both Litvinenko and his patron, Kremlin critic Boris Berezovsky, had contacts with British intelligence and that Berezovsky had given Britain sensitive information about Russia.

In a one-sentence statement, the FSB said it had opened a criminal investigation on suspicion of espionage Thursday on the basis of statements made by Lugovoi, who met with Litvinenko in London on the day he said he fell ill.

Russia has stressed that it will refuse Britain's request for the extradition of Lugovoi. The espionage case is likely to further strain relations between London and Moscow, which is angry over Britain's refusal to hand over Berezovsky and other Kremlin foes for prosecution in Russia.

It comes against the background of persistent friction between Russia and the West, and repeated Russian claims that British and other Western intelligence agencies — as well as Russians such as Berezovsky who have found refuge abroad — are seeking to weaken Russia.