Mr Anwar faces four charges alleging that he used his government position to try to cover up allegations of sodomy and adultery. He faces separate charges in connection with the alleged sex acts.

At the request of the prosecutors in the Kuala Lumpur High Court, Judge Augustine Paul altered the corruption charges to say that Mr Anwar abused his office in order to protect his reputation.

The BBC's Frances Harrison reports from Kuala Lumpur on the latest twist in the Anwar trial

This means that the prosecution can pursue the corruption charges without having to prove the sexual acts took place.

The separate charges of sodomy and adultery - illegal in Malaysia - still remain.

Malaysian law allows the prosecution to change its charges midway through a trial.

Frances Harrison: Prosecution no longer has to prove sex allegations

Mr Anwar's lawyers tried to prevent the alterations to the charges, arguing the changes would make it easier for the prosecution to prove the four corruption charges.

"They have already stripped me naked - now they are amending the charges," Mr Anwar told reporters during a recess.

A stained mattress was among the evidence presented by the prosecution

But politically, Mr Anwar's supporters see this as a victory, a sign that the prosecution cannot make the charges of sexual misconduct and sodomy stick.

The defence argued that attempts by the prosecution to prove the sex acts took place had already smeared the former minister's reputation.

But Judge Paul upheld the prosecution's argument that the commission of sexual acts was never a substantive part of the corruption case.

Defence to call Prime Minister as witness

In a further dramatic development, defence lawyers announced that they intend to call Prime Minister Mahathir Mohamad and other cabinet ministers to testify.

Although Dr Mahathir was named as a witness for the prosecution, he has not yet testified in court.

The BBC Correspondent in Kuala Lumpur, Frances Harrison, says that the prime minister is likely to be an unwilling witness, and that Mr Anwar's lawyers may be motivated by party political considerations in calling him.

Mr Anwar denies all of the charges against him.