Boris Johnson’s government was in danger of imploding on Saturday night as the work and pensions secretary, Amber Rudd, quit the cabinet and resigned the Conservative whip, saying she could not stand by while “loyal, moderate MPs” were purged from the party.

In a devastating resignation letter, she accused the prime minister of “an assault on decency and democracy” and “an act of political vandalism” for sacking 21 of her Tory colleagues for backing a parliamentary bill to stop a no-deal Brexit.

Rudd told Johnson she had joined his cabinet “in good faith accepting that no deal had to be on the table”. She added: “However, I no longer believe leaving with a deal is the government’s main objective.”

While she could see the government was “expending a lot of energy” preparing for no deal, she had not seen “the same level of intensity go into our talks with the European Union” and updates from his office had “not, regretfully, provided me with the reassurances I sought”.

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Referring to Johnson’s decision to withdraw the whip from rebel MPs, she said: “This short-sighted culling of my colleagues has stripped the party of broad-minded and dedicated Conservative MPs. I cannot support this act of political vandalism.”

Rudd’s decision follows the resignation from the cabinet of Johnson’s own brother, Jo Johnson, on Thursday and will place intense pressure on other cabinet members who are also against a no-deal Brexit, including Nicky Morgan, to follow suit.

Labour’s party chairman, Ian Lavery, said: “The prime minister has run out of authority in record time and his Brexit plan has been exposed as a sham. No one trusts Boris Johnson. Not his cabinet, not his MPs, not even his own brother.”

David Gauke, the pro-remain ­former secretary of state for justice and lord chancellor, tweeted of Rudd’s resignation: “I’m sure this has not been an easy decision. But it is brave and principled and is all about putting the national interest first.”

The Sunday Times (@thesundaytimes) Exclusive: Amber Rudd @AmberRuddHR today announces that she is resigning from the cabinet and quitting the Conservative Party over Boris Johnson’s “purge” of the party and his “failure” to pursue a deal with the EU.



Read the full story here: https://t.co/MsPibDGGR7 pic.twitter.com/iEQiIyYZlY

A source close to the group of purged former MPs said: “The wheels are coming off the Dominic Cummings Downing Street machine. The sham negotiation has been exposed for what it is – and he has no plan B. He’s single-handedly trying to turn the Conservative party into an extreme rightwing faction – and moderates are clearly not welcome.”

Rudd’s departure follows a desperate week for the new prime minister, during which a cross-party group of MPs seized control of the parliamentary timetable and succeeded in voting through a bill to block a no-deal Brexit, despite Johnson’s insistence that he would not extend the deadline for leaving the EU beyond 31 October.

Facebook Twitter Pinterest Page 1 of Amber Rudd’s resignation letter. Photograph: Amber Rudd/PA

On Saturday the prime minister was warned that he would trigger a legal and constitutional crisis that would force his resignation if he failed to obey the legislation championed by the cross-party group, including Tory rebels, mandating him to seek another extension to Brexit.

According to leaked legal advice from leading QCs sent to the shadow Brexit secretary, Sir Keir Starmer, the prime minister would be declared in contempt of court if he were to remain in No 10 while refusing to obey legislation to prevent the UK leaving the EU without a deal on 31 October. The new law is expected to gain royal assent from the Queen early this week.

The legal advice – from lawyers at Matrix Chambers – says: “If the prime minister refused to comply with this order then, while we would be in uncharted political territory, the legal position would remain clear – the prime minister would be in contempt of an order of the court and would be exposed to a full range of sanctions.”

On Saturday, as violence broke out near parliament as a pro-Brexit protest confronted a remain march, Downing Street said Johnson remained defiant.

Johnson and his advisers are this weekend exploring further legislative possibilities, including the “nuclear” option of tabling a no-confidence motion in the government and ordering Tory MPs to vote for it, in order to trigger an election. Such a motion would require only a simple majority of MPs to pass, but may not be allowed by the Speaker, John Bercow.

Announcing her decision on Twitter, Rudd said: “I have resigned from cabinet and surrendered the Conservative whip. I cannot stand by as good, loyal, moderate Conservatives are expelled.

“I have spoken to the PM and my association chairman to explain. I remain committed to the one nation values that drew me into politics.”

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Nick Boles, who quit the party in April after his soft Brexit plan failed, tweeted: “Everyone has a point beyond which they cannot be pushed. Amber Rudd has reached hers. How much more of the party he inherited will Johnson destroy before he has second thoughts or is stopped by his cabinet colleagues?

“At what point do Matt Hancock and Nicky Morgan suddenly discover that they too have backbones,” he went on. “Or are they now nailed on as the one nation window dressing for the Brexit party?”

Starmer tweeted: “Johnson government falling apart. He’s being totally found out.”

The former home secretary was dogged by questions throughout the Tory leadership contest about whether she could serve in Johnson’s cabinet if he won the race, given that his strategy would involve keeping no deal on the table during negotiations with Brussels.

She accepted the offer of continuing in her job as work and pensions secretary when Johnson formed his cabinet in July. Rudd has represented Hastings and Rye since 2010 and has one of the smallest majorities in the country, with only 346 votes separating her from her Labour rival in 2017.