LONDON (Reuters) - The lawyer for the family of former KGB agent Alexander Litvinenko, who was murdered in London in 2006, accused Britain and Russia on Tuesday of colluding to try and shut down an inquiry into his death for the sake of trade links.

Litvinenko, who had been granted British citizenship and become a vocal critic of the Kremlin, died after someone slipped polonium-210, a rare radioactive isotope, into his cup of tea at a London hotel. The fallout from his death has beleaguered diplomatic relations between London and Moscow ever since.

At a pre-inquest hearing at London's Royal Courts of Justice, Ben Emmerson, a lawyer for Litvinenko's wife, said a legal attempt by the British government to keep some information about the death confidential was part of a cover up.

Emmerson said the British government's application for a public interest immunity order, which would prevent material being disclosed in open court, was to protect business deals being struck by Prime Minister David Cameron and the Kremlin.

He said the coroner in the planned inquest, High Court judge Robert Owen, should reject the government's application.

"It is crucial, absolutely crucial, that the outcome of this hearing is to scotch once and for all ... (speculation) David Cameron is so interested in promoting trade with Russia that he's trying to close down this inquest".

(Reporting By Michael Holden, editing by Estelle Shirbon and Guy Faulconbridge)