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By Dasha Afanasieva and Michael Holden
LONDON, Dec 13 - The Russian government was most likely involved in the murder by poisoning of Kremlin critic and former spy Alexander Litvinenko in London in 2006, a British lawyer told a preliminary hearing into the death on Thursday.
The statement by Hugh Davies, an attorney acting on behalf of the inquest, is likely to enrage Russia, which has denied any involvement in the killing, and could put further strain on London's already testy relations with Moscow.
The London court also heard the former KGB agent was working for Britain's MI6 secret service when he died, suggesting the British government might come under scrutiny for failing to protect him.
Litvinenko, who had been granted British citizenship and become a vocal critic of the Kremlin, died in 2006 after someone slipped polonium-210, a rare radioactive isotope, into his cup of tea at a London hotel.
"Our assessment is that the (British) government material does establish a prima facie case as to the culpability of the Russian state in the death of Alexander Litvinenko," Davies said at Thursday's hearing, held to discuss the scope of the full inquiry into Litvinenko's death.
British police and prosecutors say there is enough evidence to charge two former KGB agents, Andrei Lugovoy and Dmitry Kovtun, but Moscow has resisted calls to extradite them.
A Kremlin spokesman and the Russian foreign ministry both declined comment and there was no reaction from either Lugovoy or Kovtun.
Russian officials have said the British focus on Lugovoy as the suspect stems from what it sees as London's anti-Russian bias and that Britain has failed to provide enough evidence.
The inquest is the latest twist in Britain's complicated ties with Russia. The two are at odds over Moscow's human rights record and foreign policy, with spy rows and tit-for-tat diplomatic expulsions dominating relations.
The full inquest into Litvinenko's death, led by Judge Robert Owen, is expected to start on May 1.
Ben Emmerson, lawyer for Litvinenko's widow, Marina, told the hearing the victim had been working for Britain's Secret Intelligence Service, known as MI6, for a number of years.
He said Litvinenko was also employed by the Spanish security services and the couple had payments from both MI6 and Spanish intelligence agencies in their joint bank account.
"At the time of his death, Mr Litvinenko had been for a number of years a registered and paid agent and employee of MI6, with a dedicated handler whose pseudonym was 'Martin'," Emmerson said.
Litvinenko would often tell his wife about the meetings he had with 'Martin', which would normally take place in central London, Emmerson said.
MI6 role gave Litvinenko "dangerous" missions - a revelation suggesting that the British government had a duty to ensure his safety.
PHONE CALL TO LUGOVOY
Adding further intrigue, he said Litvinenko called Lugovoy in 2006 from a London hospital just months before he died to cancel a trip to Spain where the two had planned to give evidence to Spanish prosecutors assessing Russian mafia activities and links to the Kremlin.
Emmerson said "Martin" had to appear before the inquest as he was "a critical witness on all issues".
He described the death as a "state sponsored assassination" and suggested the Russian state would seek "interested party" status, meaning it would have legal representation at the inquest and access to relevant documents.
Litvinenko was an associate of tycoon Boris Berezovsky, a former Kremlin insider who became a critic of President Vladimir Putin and was granted asylum in Britain.
Some in Russia, where Berezovsky is an unpopular figure often mocked on state television, have pointed figures at the exiled oligarch but Hugo Keith, Berezovsky's lawyer, denied any involvement by his client.
"It's not open to an individual to get Polonium-210. The suggestion that Mr Berezovsky is responsible is implausible," Keith said.
Davies said the secret British documents also showed no compelling evidence against Berezovsky, or against Chechen mafia or other figures who have been suggested as being involved.
He also said there was no prima facie evidence to suggest Britain had been involved in Litvinenko' death or failed to protect him.
Under British law, an inquest is held when a person dies unexpectedly to determine the cause of death.
Marina, who had been campaigning for an inquest for years, said she was hopeful. "It has already been six years," she said outside the court. "I'm looking forward to see and to know". (Writing by Maria Golovnina; Editing by Angus MacSwan)