Mr Anwar says the allegations have damaged his reputation
Former Malaysian Deputy Prime Minister Anwar Ibrahim has sued his former boss, ex-Prime Minister Mahathir Mohamad, for accusing him of being homosexual.
Dr Mahathir repeated the allegation - first made when the two leaders became rivals in 1998 - last September.
"Imagine having a gay prime minister. Nobody would be safe," he said.
Mr Anwar, who was sacked in 1998 and later jailed for corruption and sodomy, was freed in 2004 after the sodomy conviction was overturned.
Mr Anwar has always said the convictions were politically motivated after he emerged as a rival to Dr Mahathir in 1998.
In his latest writ, Mr Anwar's said that Dr Mahathir comments were "falsely and maliciously" made and caused "irreparable damage" to Mr Anwar's reputation.
ANWAR'S LEGAL BATTLE
Sept 98 - Sacked and arrested
April 99 - Jailed for six years for corruption relating to alleged sodomy
July 00 - Sentenced to further nine years for sodomy - alleged to have had sex with five men
July 2002 - Loses appeal against corruption conviction
Sept 2004 - Freed after winning appeal against sodomy conviction
"We have filed a defamation suit against Tun Mahathir today. Anwar has a very good case and we remain confident," said his lawyer Sankara Nair.
Mr Anwar said in a statement that Dr Mahathir remained "defiant in repeating his allegations. I am now left with no choice but to proceed with this suit".
Dr Mahathir - who resigned from the leadership in 2003 - told reporters in September 2005 that he sacked Mr Anwar, his one time heir-apparent, to prevent mainly-Muslim Malaysia from having a homosexual leader.
"I cannot have a person who is like that... become the prime minister. Imagine having a gay prime minister. Nobody would be safe," he said.
Mr Anwar has already successfully sued the author of a pamphlet, Fifty Reasons Why Anwar Ibrahim Cannot Be Prime Minister, which was widely circulated soon before he was sacked.
He also won damages last year from a police chief who beat him while he was in custody in 1998.
Mr Anwar is currently barred from holding office until 2008 because the corruption conviction against him still stands.