At a meeting with her Conservative Party lawmakers on Monday, she was cheered and applauded by many as she warned them that internal squabbling could pave the way for socialist opposition leader Jeremy Corbyn to take power instead. They are the first to quit over a fundamental difference on her Brexit platform, but one of them was the man in charge of implementing that policy.
David Davis and Steve Baker clearly realised that the promises they and their colleagues made about Brexit simply can not be delivered. "This is the right Brexit", she said.
The ERG's chairman, Jacob Rees-Mogg, has said he has not sent a letter to the 1922 Committee, and expects Mrs May to remain in office at least until Brexit Day in March 2019.
The Prime Minister would have been forgiven for thinking on Sunday evening that she had managed to placate unhappy Cabinet Brexiteers at Chequers on Friday. He has served as health secretary since 2012.
Britain's Prime Minister Theresa May leaves 10 Downing Street in central London after attending the weekly cabinet meeting on July 10, 2018.
Q: Why hasn't Brexit happened yet?
Mrs May said she had "full confidence" in the then-foreign secretary, despite his suggestion that the USA president Donald Trump could do a better job of negotiating Brexit than the United Kingdom government.
The British public no longer has faith in Theresa May to negotiate Brexit, with two in three saying they do not trust her to get the best possible deal, a Sky Data poll reveals.
Johnson said in his letter that May's plan to keep close economic ties with the bloc means the U.K.is heading for a "semi-Brexit" that would leave Britain with the "status of colony" within the EU.
He said many MPs saw Mr Johnson's resignation as a "selfish act".
In his resignation letter, Johnson harshly criticised the government's approach to Brexit: "Brexit should be about opportunity and hope".
He says the Brexit "dream is dying, suffocated by needless self-doubt", and that Britain is "headed to the status of a colony", having to swallow European Union writ without "any ability to influence these laws as they are made".
Political uncertainty in Britain makes the possibility of a no-deal Brexit seem far more likely in the eyes of Brussels bureaucrats.
"May is fighting for her survival", Mujtaba Rahman, a managing director at Eurasia Group, said in a research note.
Under Conservative Party rules, a confidence vote in a leader can be triggered if 15 percent of Conservative lawmakers - now 48 - write a letter requesting one.
Though negativity about immigration has fallen sharply in Britain in recent years, for this handful of MPs, leaving the European Union is all about controlling immigration-and not much else.
"Nine ministers have now resigned from the two key Brexit departments since Article 50 was triggered".