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...

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 [Tony Blair's wife], 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.