Alun Michael, a Labour home affairs spokesman, asked if there was more than had been revealed so far about the case of Paul Bennett, a New Zealander, who has spent 22 months in jail in England without trial.
Mr Michael's letter to Kenneth Clarke came amid a new twist to the story - allegations that Mr Bennett's visits in Belmarsh prison, south-east London, had been bugged, and that two men had been disturbed taking papers relating to his case from his solicitor's rubbish bins.
Colin Nott, Mr Bennett's solicitor, said two suspect devices had been found during legal visits, but a Home Office spokesman said yesterday that only one had been handed in. A police examination of that device found it to be a broken magnet from a microphone or speaker, incapable of functioning as anything other than a magnet.
Wanted in London on suspicion of obtaining a helicopter by deception, Mr Bennett claims he was arrested by South African police, held incommunicado for more than three weeks and eventually escorted to London - in defiance of a South African Supreme Court order.
He also alleges that British diplomats in South Africa furnished him with a false UK passport to ease his removal.
Mr Bennett had sought disclosure of a series of communications between Scotland Yard and South African police, which he claimed would prove he had been illegally brought to the UK. But the Home Secretary signed Public Interest Immunity certificates to suppress their public release.
Last August, Mr Bennett failed in the High Court to have the charges against him thrown out because of his alleged unlawful detention and illegal extradition. The case is to go to the House of Lords.
In his letter to Mr Clarke, Mr Michael said reports about the case 'seem so extraordinary as to encourage flights of fancy that would be more appropriate to a novel of international intrigue'. In reply Mr Clarke said it was the duty of ministers to assert claims of Public Interest Immunity in certain cases and for the court to balance the interest of immunity against public interest in the administration of justice.
But he added that the High Court had thrown out Mr Bennett's case on other grounds and the Public Interest Immunity certificate 'therefore played no part in the decision'.