Amber Rudd has quit Boris Johnson's cabinet and given up the Conservative whip after the prime minister sacked 21 rebellious Conservative MPs this week.

The work and pensions secretary blasted the firings as a "short sighted culling" and an "assault on decency and democracy."

"I cannot stand by as good, loyal moderate Conservatives are expelled," Rudd wrote in her resignation letter, which she shared on Twitter. "I cannot support this act of political vandalism."

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Amber Rudd has quit Boris Johnson's cabinet and given up the Conservative whip after the prime minister sacked 21 rebellious Conservative MPs this week. The work and pensions secretary blasted the firings as a "short sighted culling" and an "assault on decency and democracy."

"I cannot stand by as good, loyal moderate Conservatives are expelled," Rudd wrote in her resignation letter, which she shared on Twitter. "I cannot support this act of political vandalism."

Johnson removed the whip from the government ministers after they joined the opposition in voting to make a no-deal Brexit on October 31 illegal.

Rudd joined Johnson's cabinet with the understanding that the UK would only crash out of the European Union without an exit deal as a last resort, and that prospect would help it strike an agreement, she wrote. "However I no longer believe leaving with a deal is the government's main objective."

Read more: 'He will have to resign': Conservative rebel says Boris Johnson will have no choice but to leave Downing Street

Downing Street was "disappointed" by the departure of a "talented" cabinet member, a spokesperson told the BBC. However, the prime minister's office struck a defiant tone. "All ministers who joined the Cabinet signed up to leaving the EU on October 31 come what may," the spokesperson added.

The new law is intended to force Johnson to seek an extension to the Brexit deadline if he hasn't secured parliamentary backing for an exit deal by October 19. However, the prime minister is considering defying it, a senior source in his office told the Sunday Times. "If there isn't a deal by the 18 [October] we will sabotage the extension."