Simpang Renggam MP Maszlee Malik said he could not bear to be in the company of Umno leaders at Prime Minister Muhyiddin Yassin's house yesterday.
"Among them were those tainted with corruption issues, including (matters involving) the Education Ministry, where I was left to pick up the pieces," wrote Maszlee in a lengthy Facebook post today.
"I asked myself if I am ready to accept them again and allow them to be leaders of this country after they have been rejected by the people and some among them are facing justice."
Maszlee, who was education minister during the Pakatan Harapan administration, said he had consulted those close to him and sought divine inspiration before deciding at 4pm yesterday.
While arguing that he was a "party man" and believed in party discipline, Maszlee said he made the decision not to work with those he fought to defeat during the 2018 general election.
"This decision is final. National interest is above political interest," said Maszlee.
He was among the handful of Bersatu MPs who eventually backed Dr Mahathir Mohamad to be prime minister instead of Muhyiddin.
The bulk of Bersatu MPs joined forces with Umno, PAS, Gabungang Parti Sarawak and a host of smaller parties to back Muhyiddin.
The eighth prime minister was sworn in this morning.