More than 50 ministers have quit the government in less than 48 hours, saying Johnson was not fit to be in charge after a series of scandals, while dozens in his Conservative Party are in open revolt.
Brandon Lewis, the Northern Ireland Secretary, became the latest Cabinet minister to quit early on Thursday, following the resignations of the finance, health and Welsh ministers.
"I cannot sacrifice my personal integrity to defend things as they stand now," Lewis said. It is clear that our party, parliamentary colleagues, volunteers and the whole country, deserve better."
Suella Braverman, the attorney general for England and Wales, told ITV late on Wednesday she would stay in post but would stand in any future leadership contest
"I do think the time has come for the prime minister to step down," she said. "If there is a leadership contest I will put my name into the ring."
A delegation of senior ministers and a representative of Conservative lawmakers not in government went to Downing Street to see Johnson on Wednesday evening to tell him he needed to go and to make a dignified exit.
But Johnson is so far refusing to budge, and even sacked Michael Gove, a senior minister who media earlier reported had told the British leader he should quit.
"I am not going to step down," Johnson told a parliamentary committee. The Sun newspaper quoted an ally of the prime minister as saying that rebels in his party would "have to dip their hands in blood" if they wanted to get rid of him.
Johnson has suggested that he had a mandate to govern from the almost 14 million voters who voted for the Conservatives in December 2019 when he swept to power with a promise to sort out Britain's exit from the European Union after years of bitter wrangling.
He says it would not be responsible to walk away from the job in the middle of an economic crisis and war in Europe. Johnson has been a visible supporter of Ukraine following Russia's invasion in late February.
He has also refused to say if he would try to stay in the job even if he lost a confidence vote from his own lawmakers. That could come next week if they agree to change the party's rules, which only allow one such challenge a year. He narrowly won a similar vote last month.