KUALA LUMPUR, May 4 (Reuters) - The sex trial of Anwar Ibrahim was postponed indefinitely on Thursday to allow the former finance minister to appeal against a ruling preventing the defence from calling the prime minister as a witness.

High Court officials said a date would be fixed for the trial to reconvene after the Court of Appeal rules on an appeal by the defence challenging the trial judge's decision to spare Prime Minister Mahathir Mohamad from testifying.

Anwar's lawyers said last week that they would appeal against High Court Judge Arifin Jaka's decision to reject Anwar's request to call Mahathir as a witness in the trial, which has been under way for 11 months with no end in sight.

Anwar, currently serving a six-year jail sentence for corruption, is standing trial on one count of sodomy punishable by up to 20 years in jail and whipping.

The former deputy prime minister has denied any wrongdoing and said Mahathir's testimony was crucial in his effort to convince the judge that he was framed as part of a political conspiracy by the prime minister and associates.

Mahathir has denied any plot against Anwar and accused his former protege of trying to topple his government.

No date has been fixed for the Court of Appeal to take up the case, defence lawyers said.

"The date cannot be fixed until we file a petition of appeal stating the grounds of challenge," lawyer Pawancheek Marican told reporters. "We have filed the notice of appeal. But we have yet to file the petition of appeal."

Judge Arifin had adjourned the trial last Wednesday pending a decision on the appeal. But two days later the judge called for the trial to continue with other witnesses.

The move prompted objections from both the defence and prosecution, and the judge agreed to halt proceedings. But he then called for the trial to reconvene on Thursday to hear evidence on Anwar's defence of alibi.

The judge was not in the courtroom on Thursday when court officials announced that the trial had been postponed.

It was not clear when the defence would file the petition of appeal or when the Court of Appeal would take up the case.