The biggest party in Malaysia’s ruling alliance declared Tuesday that embattled Prime Minister Muhyiddin Yassin has lost the right to govern because a number of its lawmakers have signed declarations withdrawing their support.

The announcement by the United Malays National Organization was followed by the resignation of Energy and Natural Resources Minister Shamsul Anuar Nasarah, who is from the party.

Mr. Muhyiddin took power in March 2020 after initiating the collapse of the former reformist government that won 2018 elections. His party joined hands with UMNO and several others to form a new government but with a razor-thin majority.

UMNO, which has 38 lawmakers, earlier said it would pull its support for Mr. Muhyiddin, but some party lawmakers still back the prime minister.

UMNO President Ahmad Zahid Hamidi declared Tuesday that Mr. Muhyiddin’s government has fallen because enough of the party’s lawmakers have signed declarations withdrawing their support for it. He said at a news conference that the declarations were presented to the king to show Mr. Muhyiddin has lost majority support and the legitimacy to rule.

He didn’t say how many party members signed the declarations, but there were at least 10 other UMNO lawmakers at the news conference. Mr. Muhyiddin had only a two-seat majority last year, and Parliament hasn’t been able to hold no-confidence votes since then.

There was no immediate reaction from the king or Mr. Muhyiddin.

Local media reported that Deputy Prime Minister Mr. Ismail Sabri, an UMNO member, along with the attorney general, several Cabinet ministers and other politicians were seen entering Muhyiddin’s residence late Tuesday after the UMNO announcement.

Muhyiddin is now facing increasing pressure and there is uncertainty about his political future, said Ms. Bridget Welsh of Malaysia’s University of Nottingham, an expert on Southeast Asian politics.

Ms. Welsh said Mr. Muhyiddin may try to fight back but has limited choices. She said it’s up to the king now to assess UMNO’s claim and decide on the next step.

The opposition filed a new no-confidence motion against Muhyiddin last Thursday after the King Sultan Abdullah Sultan Abdullah rebuked the government for misleading Parliament on the status of ordinances issued during a coronavirus state of emergency.