Russia has sent Britain a letter officially turning down a request to extradite Andrei Lugovoy, the main suspect in the murder of Alexander Litvinenko.
Russian officials had previously told reporters they would not extradite Mr Lugovoy, but Britain was awaiting an official response.
Moscow's response is in accordance with the Russian constitution, which forbids the extradition of a Russian citizen to a foreign state.
British prosecutors say they want to try former Russian state security agent Mr Lugovoy for the murder of Mr Litvinenko, another Russian ex-agent who died in London last November from radiation poisoning.
Mr Litvinenko's associates accused Russian President Vladimir Putin of having a hand in the murder. The Kremlin dismissed that accusation as nonsense.
The case has caused a diplomatic rift between Moscow and London, two former Cold War foes now linked by billions of dollars of investment and business interests.
Britain says it expects Russia to give up Mr Lugovoy for trial, while Moscow says the British government is participating in a campaign to blacken Russia's reputation.
