(Adds comment from British ministry in paragraph 10)
MOSCOW, July 5 Russia on Thursday formally rejected a British request to extradite the main suspect in the murder by poison of Russian emigre Alexander Litvinenko, deepening a diplomatic feud.
Russian officials had already told reporters they could not extradite ex-state security agent Andrei Lugovoy, but Britain had been waiting for an official response to its request.
In a statement, the Prosecutor-General's office cited article 61 of the Russian constitution, which it said barred the extradition of Russian citizens.
"It is not possible to meet the British request to hand over Andrei Lugovoy," said the statement, adding that the British interior ministry had been informed.
Litvinenko, a former officer in Russia's Federal Security Service, fled to Britain and became a critic of President Vladimir Putin. He died in a London hospital last November after being poisoned with radioactive polonium-210.
He said in a statement read out by associates after his death that Putin had had a hand in his murder. The Kremlin dismissed the charge as nonsense and accused Britain of harbouring emigres bent on blackening Russia's name.
A spokesman for Litvinenko's widow, Marina, said Russia's refusal to extradite Lugovoy was further evidence of a cover-up.
"She thinks this is another piece of evidence that the Russian government was behind this murder," said Alex Goldfarb. "She expects a reaction from the British government because this was a terrorist attack and not a simple murder."
The British embassy in Moscow said it was "not yet aware of any official response to the extradition request".
A spokeswoman for Britain's Home Office (interior ministry) said it did not comment on individual extradition cases.
Lugovoy met his business partner Dmitry Kovtun and Litvinenko in the bar of London's Millennium Hotel hours before Litvinenko fell ill.
Police found a trail of radiation matching the movements of Kovtun and Lugovoy.
Lugovoy has denied any guilt, saying he believed Litvinenko had been killed by British intelligence.
Russian prosecutors said they were prepared to assist the British investigation, and could prosecute Lugovoy themselves if Britain provided sufficient evidence of his guilt.
Some analysts say the row between Britain and Russia could threaten billions of dollars in trade and investment.
In April, Russian officials and executives pulled out of the Russian Economic Forum in London because, Russian media reported, the Kremlin instructed them to stay away.
London is the financial centre of choice for big Russian companies seeking listings, and BP's (BP.L) Russian vehicle TNK-BP is a player in Russia's energy sector.
In another source of tension, Moscow is pressing London to extradite Russian tycoon and Kremlin critic Boris Berezovsky, a pall bearer at Litvinenko's funeral in London, on embezzlement charges.
A British court has refused to hand him over, arguing he would not have a fair trial in Russia.
(Additional reporting by James Kilner)