(Corrects headline, recasts lead, para 2, adds paras 3-5)

LONDON, May 25 (RIA Novosti) - British prosecutors said Friday they had filed an extradition request with the Home Office, to be sent to Russia, for a former FSB guard accused of murdering ex-spy and Kremlin critic Alexander Litvinenko.

The Director of Public Prosecutions Ken Macdonald said the formal request for the extradition of Andrei Lugovoi had been made in line with the European Convention on Extradition and also contained an arrest warrant and detailed account of evidence gathered against the businessman.

Russia's top prosecutor, Yury Chaika, reiterated earlier Friday, citing the Russian Constitution, that Lugovoi would not be extradited to Britain, but would stand trial in Russia if his complicity was substantiated.

"If our experts find British suspicions with regard to Lugovoi credible, he will be tried in a Russian court of law," he said. "But I repeat we will not extradite the Russian national."

Lugovoi denied any role in the murder of Litvinenko, who received UK citizenship a few weeks before his fatal radioactive poisoning last November, saying the charges were political.