KUALA LUMPUR, Malaysia (CNN) -- Sacked Malaysian Deputy Prime Anwar Ibrahim, who is facing a dramatic sex and corruption trial, suffered a blow in a separate court case, the official Bernama news agency reported.

The agency said Kuala Lumpur's High Court had rejected Anwar's claim that his sacking by Prime Minister Mahathir Mohamad in September was unconstitutional.

Anwar's lawyers had argued that Mahathir violated the constitution by allegedly failing to obtain the king's consent before sacking the former deputy prime minister and finance minister.

They told the court that only Malaysia's king, known as Yang di-Pertuan Agong, could dismiss a minister and Mahathir had violated the constitution by sacking Anwar without the monarch's assent.

In his lawsuit, Anwar had asked the court to declare that he was still Cabinet minister, and sought costs and undetermined relief.

Mahathir recently told parliament that he had conveyed his decision to the king and the monarch had expressed no objections to Anwar's sacking.

Meanwhile, the defense in Anwar's trial said on Wednesday it had an audio tape supporting Anwar's claims he was the victim of a political conspiracy.

Defense counsel Gurbachan Singh read to the capital's High Court portions of a transcript of the tape which he said recorded the voice of Ummi Hafilda Ali, the woman who in 1997 accused Anwar of sex crimes.

Gurbachan told the court that the tape was a recording of a conversation between Ummi and a male business associate named Sng Chee Hue in London last June.

On the tape, a woman's voice discusses Prime Minister Mahathir Mohamad, saying he had congratulated the author of a book, "Fifty Reasons Why Anwar Ibrahim Cannot Become Prime Minister," which accused Anwar of numerous crimes.

"I put it to you this was said by you to Dato Sng (Chee Hue)," Gurbachan said to Ummi, who was on the witness stand for the third day. She said she did not remember the conversation.

Judge Augustine Paul said he would decide on Thursday whether to allow the tape to be admitted as evidence in the trial, which has exposed deep divisions between supporters of Mahathir and Anwar.

Anwar has pleaded not guilty to five counts each of corruption and sodomy. His trial, at the center of civil unrest in Malaysia, is in its seventh week in the capital's High Court.

Reuters contributed to this report.