Amber Rudd is resigning from cabinet and the Tory party

The work and pensions secretary says she is quitting because there is “no evidence” Johnson is seeking a deal – despite his claims otherwise.

“At the moment there’s a lot of work going into no deal, and not enough into getting a deal.”

In an interview with the newspaper, Rudd said Johnson’s sacking of 21 rebel MPs earlier in the week was an “assault on decency and democracy”.

“This short sighted culling of my colleagues has stripped the party of broad-minded and dedicated Conservative MPs,” she wrote in her letter to Boris Johnson.

She said that she “cannot stand by as good, loyal moderate conservatives are expelled”.

Rudd has now “surrendered” the Tory whip, and will run as an independent Conservative in Hastings and Rye at the next election where she has just a 346 vote majority.

Best for Britain chief executive Naomi Smith said it was the “right decision”.

She said: “Amber Rudd has finally made the right decision and chosen to resign from Boris Johnson’s cabinet, joining Jo Johnson in putting country before career.

“The prime minister has managed the incredible feat of turning a mainstream political party into a fringe organisation in a matter of days.”

Former Conservative MP and minister 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?”

Rudd had been criticised by Remainers for a lack of consistency over Boris Johnson’s leadership and a no-deal Brexit in recent weeks.

She positioned herself as a fierce opponent to Boris Johnson leading the country and the UK leaving the EU without a deal.

But after backing Jeremy Hunt, she accepted no-deal Brexit was a possibility before she took up a post in Boris Johnson’s cabinet.

She told Newsnight: “The no-deal option is really a contingency plan. I describe it as wearing a seatbelt when you’re driving a car.

“Just because you have a seatbelt on doesn’t mean you want to have a crash.

“That’s how I view no-deal Brexit. It’s there in case it happens, but very much at the centre of government policy is getting a deal.”

Rudd also recently refused to answer questions over Boris Johnson’s plans to prorogue parliament, despite saying that “the idea of leaving the EU to take back more control into parliament and to consider the idea of closing parliament to do that is the most extraordinary idea I’ve ever heard.”

She continued: “It is a ridiculous suggestion to consider proroguing parliament. For a start it would involve approaching the Queen and nobody should consider doing that.”