Embattled British Prime Minister Boris Johnson suffered a new blow Saturday when a Cabinet minister resigned, saying she did not believe Johnson was sincere about getting a Brexit deal with the European Union. Work and Pensions Secretary Amber Rudd said in a letter to the prime minister that she was resigning because "I no longer believe leaving with a deal is the government's main objective."

Rudd also slammed Johnson's expulsion of Conservative lawmakers who oppose his Brexit plans, calling it "an assault on decency and democracy."

This week Johnson kicked 21 lawmakers out of the Conservative group in Parliament after they sided with the opposition to pass a law designed to prevent Britain from leaving the European Union next month without a divorce deal.

Britain's Work and Pensions Secretary and Women's minister Amber Rudd leaves the Cabinet Office on Whitehall in central London on September 3, 2019. Niklas Halle'n / AFP/Getty Images

Those expelled included the longest-serving Conservative in Parliament, Ken Clarke, and Nicholas Soames, grandson of World War II Prime Minister Winston Churchill.

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"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," wrote Rudd, who said she is also quitting the Conservative group and will sit as an independent.

Johnson says Britain must leave the EU as scheduled October 31 even if there is no divorce agreement with the bloc. But his plan is meeting fierce resistance, including from some members of his own party.

This week the prime minister's younger brother, Jo Johnson, also quit the government over Brexit, saying he had been "torn between family loyalty and the national interest."

Johnson is seeking a snap election on October 15 as a way to break the deadlock over Brexit, but lawmakers last week rejected his call for an early poll.