The Prosecutor General's Office said in a statement that the Constitution bars Russian nationals from being extradited for trial, and said its cooperation with British law enforcement officials would be in line with Russian law and international treaties.

Andrei Lugovoi, also a former security officer, was accused by the Crown Prosecution Service on May 22 of murdering Litvinenko, who had fled to the U.K. in 2000, claiming his life was in danger after refusing an order to assassinate his patron Boris Berezovsky, a powerful Russian tycoon who fell out with the Kremlin and fled to the U.K. in 2001, where he was granted political asylum.

Though Litvinenko is thought to have been poisoned with radioactive Polonium-210, no official autopsy report has so far been made available. Lugovoi has vehemently denied the accusations against him claiming they are politically motivated, and in turn accused Litvinenko and Berezovsky of spying for MI6. - RIA Novosti