Mahathir Mohamad, who long dominated Malaysian politics as prime minister and leader of its governing party, said on Monday that he was quitting the organization because he was embarrassed by its role in protecting Prime Minister Najib Razak from accusations of corruption.
Mr. Mahathir, 90, said that he had no plans to start a new party to rival the one that he was leaving, the United Malays National Organization, and that he was not asking others to quit.
“I want to leave UMNO because it is no longer UMNO,” he said. “It is a party dedicated to protecting Najib. I can’t be a member of such a party.”
Mr. Najib has denied any wrongdoing.
The prime minister, who was scheduled to travel Monday night for meetings in Saudi Arabia, was asked by reporters in the capital, Kuala Lumpur, about Mr. Mahathir’s departure from the party. The prime minister smiled but made no comment.
Mr. Mahathir, who for months has been critical of Mr. Najib — his onetime protégé — has parted ways with United Malays twice before.
In 2008, he quit in a bid to push his handpicked successor, Prime Minister Abdullah Ahmad Badawi, to resign. After Mr. Badawi stepped down the next year, Mr. Mahathir rejoined the party.
In 1969, Mr. Mahathir was expelled from the party after he called for the resignation ofPrime Minister Tunku Abdul Rahman over his handling of race riots. After Prince Abdul Rahman stepped down the following year, Mr. Mahathir rejoined the party.
Mr. Mahathir served as prime minister of Malaysia and president of United Malays from 1981 to 2003.
“I feel embarrassed that I am associated with a party that is seen as supporting corruption,” he told reporters in the administrative district of Putrajaya. “It had caused me to feel ashamed.”
Mr. Najib faces accusations, first reported byThe Wall Street Journal and The Sarawak Report, that he received millions of dollars from the sovereign wealth fund 1Malaysia Development Berhad, known as 1MDB, whose advisory board he leads.
One transfer, for about $13 million, is believed to have originated in SRC International, a former subsidiary of 1MDB that is now owned by the Finance Ministry, which Mr. Najib controls.
Mr. Najib has said little about the transfers.
But since the scandal came to light, some of Mr. Najib’s critics inside the government have been removed, investigations into his conduct have been curbed, and he has consolidated his power in UMNO. His administration has also banned some online news outlets, including The Sarawak Report.
Last week, Mr. Najib’s allies in the party suspended its deputy president, Muhyiddin Yassin, for not properly supporting the prime minister.
Mr. Muhyiddin, who was also removed last year from his post as deputy prime minister, responded by saying that he had seen “proof” that the prime minister had engaged in criminal conduct. He has not produced the evidence he cited.