KUALA LUMPUR: The son of former Malaysian leader Mahathir Mohamad quit as chief minister of a northern state on Wednesday, forced out by loyalists of the country's scandal-plagued Prime Minister Najib Razak as he battles for political survival.

The ousting of Mukhriz Mahathir as chief minister of Kedah tightens Najib's grip over the United Malays National Organisation (UMNO), weakening a rival faction in the ruling party.

Mahathir, Malaysia's longest-serving prime minister, has been Najib's fiercest critic, repeatedly calling on him to step down over a multi-billion dollar scandal at state fund 1Malaysia Development Berhad (1MDB) and over deposits of $681 million in the prime minister's private bank account.

Mukhriz has also been critical of Najib, and UMNO lawmakers in Kedah aligned to the prime minister retaliated two weeks ago, declaring that Mukhriz had lost majority support in the state assembly.

"The true reason for this action against me is due to my criticism of the prime minister, as he himself has admitted," Muhkriz said in a statement read out to a news conference.

"My criticism was on the 1MDB scandal and the 2.6 billion ringgit donation and the increasing burden on the people caused by GST (Goods and Services Tax)," Mukhriz said.

The internal strife has damaged UMNO, which has led all of the country's ruling multi-ethnic coalitions since Malaysia's formation in 1957.

UMNO Sabah state liaison committee deputy chairman Salleh Said Keruak told Reuters that the party would have risked losing control of Kedah if Najib hadn't replaced Mukhriz, as state party leaders no longer had confidence in the chief minister.

"So the party president has no choice but to replace the chief minister. If not the state is going to fall," Salleh said.

The greatest threat to Najib's political survival could still come from within UMNO, if members decide he has become unelectable.

The opposition bloc split after its leader, Anwar Ibrahim, was jailed a year ago on a sodomy conviction that many observers said was politically motivated.

The febrile state of the Southeast Asian nation's politics prompted the prime minister's younger brother, Nazir Razak, chairman of the CIMB Group, to draw parallels with HBO's mediaeval fantasy series "Game of Thrones".

"The parallels with GoT continue. The future terrifies me: I just can't see how our institutions can recover, how our political atmosphere can become less toxic," Nazir wrote in an Instagram post on Saturday.

Najib sacked deputy prime minister Muhyiddin Yassin and several other cabinet members for openly questioning his handling of 1MDB, weeks after the scandal erupted in the middle of last year. He also removed Attorney-General Abdul Gani Patail, who created a special task force to investigate 1MDB.

The Wall Street Journal reported last July that investigators probing 1MDB had identified funds worth 2.6 billion ringgit that had been transferred directly to the prime minister's bank accounts.

Najib has denied any wrongdoing and says he did not take any money for personal gain. Malaysia's new attorney-general, Apandi Ali, last week closed all investigations of Najib, despite the anti-graft agency recommending that Najib be charged with criminal misappropriation.

The prime minister now faces renewed pressure after international investigators revealed they had grounds to probe 1MDB for corruption.

Apandi said the money deposited into Najib's accounts was a private donation from Saudi Arabia's royal family and that no further action needed to be taken on the matter.