Human Rights Watch has called upon the Malaysian government to release prominent opposition political figure Anwar Ibrahim, who has been embroiled in what it says is a politically motivated gay sex case for the last 15 years.

Citing Anwar's failing health and the lack of medical facilities offered to him while in incarceration, HRW said the confidence in Malaysian justice system gets a knock every day Anwar is in prison.

"Malaysia's conviction of Anwar Ibrahim was politically motivated, and he's already suffered through a year in prison from this travesty of justice," Phil Robertson, deputy Asia director of Human Rights Watch, said.

Anwar is currently serving a fresh 5-year-term awarded to him on sodomy charges. On February 10 last year, the Federal Court upheld a guilty verdict awarded to him under the country's sodomy laws.

Anwar, a former deputy prime minister and the deemed political successor of Mahathir Mohammed, was arrested in 1998 on corruption and sodomy charges. He was sentenced to six years imprisonment the next year, but later he was awarded another 9-year prison term.

He was released from solitary imprisonment after the Supreme Court overturned the guilty verdict in 2004.

Anwar confronted his former mentor Mahathir politically with his multi-party Pakatan Rakyat opposition alliance but his elected political career could not be revived as Malaysian law prevented anyone who served more than one year in prison for a criminal offence from holding elected office for five years.

He was arrested again in July 2008, after a former political aide revealed Anwar had consensual sex with him. Following lengthy court proceedings, he was handed a five-year prison term in last February.

"Anwar's conviction and imprisonment removed a major political threat to the government of Prime Minister Najib Abdul Razak," HRW said.

"The conviction effectively removed a charismatic opposition leader, already in his late 60s, from politics for a minimum of 10 years."

The rights group said Malaysia should release Anwar and repeal the controversial sodomy law, noting that so many activists in Malaysia are facing politically-motivated charges for sedition and other crimes.