Newsclip from the Wall Street Journal on Tun Mahathir’s resignation from UMNO. The article can be viewed here.

KUALA LUMPUR, Malaysia—Malaysia’s former leader Mahathir Mohamad is quitting the country’s ruling party, saying he didn’t want to be associated with a group that he said is seen as supporting corruption under the leadership of Prime Minister Najib Razak.

The country’s longest-serving leader has been a staunch critic of Mr. Najib. He remains an influential figure in Malaysia for the way he helped engineer decades of economic growth.

Dr. Mahathir, 90 years old, made his remarks Monday to reporters in the country’s administrative capital, Putrajaya, days after the United Malays National Organization, which Mr. Najib leads, suspended a deputy party president who had called on authorities to step up investigations into allegations of corruption connected to state investment fund 1Malaysia Development Bhd., which Mr. Najib established in 2009.

MALAYSIA’S 1MDB DECODED

“I want to leave UMNO because it is no longer UMNO,” Dr. Mahathir said, referring to the ruling party by its acronym. “It is a party dedicated to supporting Mr. Najib, to protecting Mr. Najib, to upholding whatever it is that he does, including some of the wrong things that he has done. I cannot be a member of such a party.”

The party has yet to respond to Dr. Mahathir’s resignation. UMNO secretary-general Tengku Adnan Tengku Mansor couldn’t immediately be reached for comment.

The government’s communications minister, Salleh Said Keruak, also an UMNO member, said in a blog post Monday that, “If Dr. Mahathir wants to continue to attack UMNO, then he should do so outside UMNO. That would be the noble and ethical thing to do.” Mr. Najib’s office didn’t respond immediately to a request for comment.

Dr. Mahathir’s resignation alone appears unlikely to ignite a revolt within the ruling party over a scandal involving graft allegations at the multibillion-dollar state investment fund, known as 1MDB.

“I doubt his resignation will have an impact unless he is able to trigger even more resignations and protests,” said Wan Saiful Wan Jan, chief executive of Kuala Lumpur-based think tank Institute for Democracy and Economic Affairs. “If this remains a lone protest it will not make any difference. In order to have an impact, it has to snowball into a bigger protest. Almost nobody in the party is in the mood. They want to maintain the status quo.”

Dr. Mahathir served as Malaysia’s prime minister for 22 years. He has repeatedly urged Mr. Najib to resign since The Wall Street Journal reported last summer that government investigators had found that payments of almost $700 million entered Mr. Najib’s personal bank accounts via banks, companies and other entities linked to the state investment fund, known as 1MDB. The probe didn’t name the source of the funds or say what happened to the money.

Mr. Najib has denied wrongdoing or taking money for personal gain. The 1MDB fund also has denied wrongdoing or taking money for personal gain, and has said it was cooperating with investigations. Malaysia’s Attorney General Mohamed Apandi Ali said the funds transferred into Mr. Najib’s private accounts were a legal political donation from Saudi Arabia.

Dr. Mahathir, a physician before entering politics, previously resigned from UMNO in 2008 over his dissatisfaction with the leadership of then-leader Abdullah Ahmad Badawi.

Some members of UMNO privately have said Dr. Mahathir turned against Mr. Najib long before the 1MDB scandal broke.

Dr. Mahathir, though, this time said he is abandoning UMNO because it has lost its sense of purpose after governing the country uninterrupted since independence from Britain in 1957. Speaking with reporters, he described how the party was founded in 1946 to help make the argument for independence and improve the welfare of the country’s majority ethnic-Malay Muslims.

Dr. Mahathir said UMNO had gone a long way toward achieving that goal, but that the party today under Mr. Najib’s leadership was unrecognizable.

He complained about a long series of perceived problems, including slowing economic growth and recent moves to sideline government critics such as Muhyiddin Yassin, the UMNO official who was suspended last week from the party and was earlier fired by Mr. Najib for questioning his leadership.

“UMNO is no more,” he said. “Today we have Party Najib.”

A series of local probes began last year into the 1MDB scandal, including by the police, auditor general, antigraft agency and the central bank.

Other jurisdictions where 1MDB money also allegedly flowed are probing the matter, including the U.S., Singapore, Switzerland, Hong Kong and, according to people familiar with the matter, Abu Dhabi. In January, the Swiss attorney general said a total of $4 billion had been misappropriated from 1MDB through complex financial structures.

Mr. Najib’s government has aggressively pushed back against accusations of wrongdoing.

—Celine Fernandez in Kuala Lumpur and James Hookway in Bangkok contributed to this article.

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