Secret evidence given by the original prosecutor in Roman Polanski's 33-year-old sex case will remain sealed, a judge ruled.

Los Angeles Superior Court judge Peter Espinoza, turning down a request from the Oscar-winning film director's lawyers to unseal transcripts, said there was only one circumstance in which the material would become public - if Mr Polanski returned to the US for a hearing and the retired prosecutor, Roger Gunson, was unavailable to give evidence in person.

"We continue to be bogged down in the question of whether he will ever return," said Judge Espinoza.

During a brief hearing the judge presented lawyers with a tentative denial before hearing arguments.

Mr Polanski lawyer Chad Hummel tried to change his mind, saying the testimony was potentially crucial to proving Mr Polanski's claim of misconduct by the judge in his case, who has since died.

He argued that Swiss authorities should have the evidence to assist in their decision whether to extradite Mr Polanski who is under house arrest. But the judge noted in a written ruling that Swiss justice ministry officials said the transcripts "are irrelevant for the extradition proceedings".