Russia ? Russia has refused Britain’s request to extradite a businessman accused in last year’s fatal poisoning of former KGB agent Alexander Litvinenko, prosecutors said Thursday.

Prosecutor General Yuri Chaika’s office said its refusal to turn over Andrei Lugovoi was based on a constitutional ban on turning Russian citizens over to foreign countries, as well as a European convention that allows signatories to refuse to extradite their nationals.

Extraditing Lugovoi “is not deemed possible,” the office said in a statement, adding that it had informed Britain’s Home Office of the decision.

In London, the Home Office said it would not comment on individual cases. Asked if the British government had received formal notification that Russia was refusing to allow the extradition of Lugovoi, Michael Ellam, Prime Minister Gordon Brown’s spokesman, said: “My understanding is no.”

In May, Britain accused Lugovoi, a former KGB agent-turned-businessman, of involvement in the killing of Litvinenko, who died in a London hospital in November from a fatal dose of the radioactive substance polonium-210.