By Michael Holden

LONDON (Reuters) - Britain's long-awaited inquest into the death of Russian spy Alexander Litvinenko has been delayed again and will not begin until October, the coroner overseeing the case said on Thursday.

Robert Owen, a senior judge acting as coroner, said Britain's main suspect, Andrei Lugovoy - a former security agent who is now a member of Russia's parliament - was wrong to refuse to cooperate with his investigation.

Litvinenko, 43, who had been granted British citizenship, died after someone slipped polonium-210, a rare radioactive isotope, into his cup of tea at a plush London hotel in 2006.

In a deathbed statement, he accused Russian President Vladimir Putin of ordering his murder, a claim Russia has denied. British police and prosecutors say there is evidence to charge former KGB agents Lugovoy and Dmitry Kovtun with murder.

An inquest, which under British law must be held when someone dies unexpectedly to determine the circumstances, was due to start on May 1, having already been postponed for years.

But after hearing that evidence, including 60 volumes amounting to some 15,000 pages of documents from the Investigative Committee of Russia, would not be ready in time, Owen said he would postpone his inquiry until October 2.

"There can be no departure from that date," Owen told a pre-inquest hearing at London's Royal Courts of Justice.

Addressing Litvinenko's widow Marina, he said: "I'm acutely aware that this will come as a great disappointment to you that this matter will be deferred for a further five months."

Ties between Britain and Russia fell to a post-Cold War low following Litvinenko's death, but British Prime Minister David Cameron has sought better relations since taking office in 2010.

The two countries' foreign and defense ministers met in London on Wednesday in the latest effort to improve diplomatic links, after which no public mention of Litvinenko was made.

SECRET MATERIAL

Another pre-inquest hearing last month heard that Britain had requested that material relating to the death should now be kept secret in the interests of national security.

That prompted accusations from the lawyer for Litvinenko's family that Britain was trying to cover up his work for its MI6 intelligence service, as well as material that showed Moscow was behind his death to protect Russian trade deals.

On Tuesday Lugovoy, who denies any involvement in the death, said he would no longer cooperate with the inquest, accusing the British government of concealing evidence.

"I have come to the conclusion that the British authorities will not give me a chance to prove my innocence," said Lugovoy, 46, who had appointed lawyers to represent him at the inquiry.

However, Owen said Lugovoy was "simply wrong" in his assessment. He told the hearing he had not yet decided whether any evidence would be withheld and said any such material could not then form any part of his conclusions.

"It would appear from that assertion that he does not understand the concept of the independence of the judiciary...," said Owen, adding he would give his conclusion on the British government's request in the next two weeks.

After the hearing, Marina Litvinenko said she was not surprised by the delay to the inquest nor Lugovoy's decision.

"I still believe in British justice," she told reporters.

However, Alexander Goldfarb, a close friend of Litvinenko, said he suspected Lugovoy had only cooperated in the first place so he could see the evidence which the police had against him which was made available to all parties involved in the inquest.

"His decision to pull out should also be viewed in that context," he said.

(Editing by Alistair Lyon)