LONDON/MOSCOW, May 22 (RIA Novosti) - British prosecutors announced Tuesday that they would seek the extradition of Russian businessman Andrei Lugovoi, to charge him with poisoning Kremlin critic Alexander Litvinenko last November.

A source at the Russian prosecution service ruled out the possibility of extradition, saying it is against the country's constitution.

The head of the UK's Crown Prosecution Service, Ken Macdonald, said: "I have today concluded that the evidence sent to us by the police is sufficient to charge Andrei Lugovoi with the murder of Mr Litvinenko," and vowed to "take immediate steps to seek the early extradition of Andrei Lugovoi" to bring him swiftly to trial in London.

But a source in Russia's Prosecutor General's Office said: "Under the Russian Constitution, Russian citizens cannot be extradited to be tried abroad. Lugovoi is a Russian citizen."

Russian lawyers representing Lugovoi, who is being treated by Russian investigators as a witness in the poisoning case, said they have not been officially notified of the charges.

"I know nothing [about the charges]. If there are any, we should be formally notified. So far, we have not received any such documents," Andrei Romashov, one of Lugovoi's lawyers, said.

Litvinenko, a former security services officer who received UK citizenship a few weeks before his fatal poisoning on November 1 last year, accused Russia's president of orchestrating his death, a charge that Vladimir Putin dismissed as ridiculous. The incident has strained relations between Moscow and London.

Lugovoi has vehemently denied complicity in the murder.