MI6 has been ordered by a judge to appear at a special public hearing over the case of one of its wartime superspies, whose file is buried in the archives of the headquarters of the Secret Intelligence Service.

In an unprecedented development, MI6 will have to explain its policy on keeping all its files locked away from the public gaze. Unlike MI5, which has been releasing large batches of its wartime records to the National Archives in Kew, MI6 has kept all its files secret.

A challenge to its policy of secrecy has been made by the nephew of Paul Rosbaud, an Austrian physicist and metallurgist who spied for Britain in the Second World War and provided crucial intelligence on German attempts to build a uranium atomic bomb.

Rosbaud was regarded as one of the most important spies of the Second World War and was codenamed “Griffin”.

Vincent Frank-Steiner, his nephew, has been trying for years to persuade MI6 to hand over the files on his uncle, and has Cherie Booth, QC, acting for him. “I want to know more about the hero in our family,” he told The Times yesterday.

A breakthrough in his campaign occurred last Friday in the private chambers of Mr Justice Sir Michael Burton, vice-president of the Investigatory Powers Tribunal. He ruled that a public hearing should be held, during which he would listen to arguments about whether secret MI6 files should be released.

John Halford, Dr Frank-Steiner’s solicitor from Bind-man & Partners, said that the hearing, which is due in about three months, would have to be based on a “hypothetical situation because MI6 neither confirms nor denies that it has a file on Paul Rosbaud”.

Dr Frank-Steiner said: “We are very proud of our relative but the full story of what he did for British intelligence in the war has never been told. One book has been written of his exploits, but MI6 is the only organisation that knows the full facts and we, as a family, would like to know what he achieved.”

What is known is that Rosbaud offered his services to Britain because he was horrified by the ambitions of the Nazi regime. At the time he was the editor of a leading scientific journal. His offer to be a spy was accepted by Frank Foley, Britain’s chief wartime spymaster in Germany.

Rosbaud passed on secret information about Germany’s research into jet aircraft, radar and the V2 and V1 rockets, as well as Hitler’s plan to develop a nuclear bomb.

Dr Frank-Steiner said he believed that inaccurate speculation had developed about his uncle’s espionage career.

Mr Halford said: “A complete and accurate picture will only emerge if the files are made public, as they can be under the Public Records Act, given the passage of time.”

Under MI6 policy, all files held on agents are subject to a national-security exemption, although an official history of the Secret Intelligence Service is due to be published by Keith Jeffrey, Professor of British history at Queen’s University, Belfast, in 2010.

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