(CNN) Senior Conservative lawmaker Amber Rudd has resigned as the UK's Secretary of State for Work and Pensions, accusing Prime Minister Boris Johnson of an assault on democracy.

Her resignation will come as a blow to Johnson, after his brother Jo Johnson resigned on Thursday saying he was "torn between family loyalty and the national interest." Former Conservative MP Phillip Lee also defected to the Liberal Democrats during Prime Minister's Questions on Wednesday.

Rudd posted her letter to the Prime Minister on Twitter Saturday night, saying she could not "stand by as good, loyal moderate Conservatives are expelled" which she called an "assault on decency and democracy."

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. pic.twitter.com/kYmZHbLMES

Having Rudd join Johnson's Cabinet was considered a coup for the new Prime Minister, and she was considered the most important Remainer in his government.

Speaking on the BBC's Andrew Marr Show on Sunday, Rudd said she had "not seen enough work going into actually trying to get a deal."

"When I asked Number 10 for a summary of what the plan was for actually getting a deal, I was sent a one-page summary," Rudd said.

She said she believes Johnson is "trying to get a deal with the EU," but added that there had been a considerable amount more of resources preparing instead for a no-deal Brexit .

"It's like 80%-90% of government time going into preparing for no-deal and the absence of actually trying to work to get a deal is what has driven 21 of my colleagues to rebel, and I need to join them," Rudd told Marr.

"It's like 80-90 percent of government time going into preparing for no-deal"



Amber Rudd says absence of govt trying to get a #Brexit deal is behind her decision to quit the Cabinet and stand by 21 Tory colleagues expelled from Conservative party #Marr https://t.co/kiy3X7g7KH pic.twitter.com/IzRzagrhdJ — BBC Politics (@BBCPolitics) September 8, 2019

In her letter to the PM on Saturday, Rudd told Johnson that she "no longer" believed "leaving with a deal is the government's main objective."

"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," she said.

Johnson 'will obey the law,' Chancellor says

After British lawmakers voted on Wednesday to pass a bill aimed a preventing a no-deal Brexit, Johnson said he would call for snap elections on October 15 rather than be forced to request another "pointless delay."

The bill, which was passed by the House of Lords on Friday will become law on Monday after Queen Elizabeth II approves it.

Chancellor Sajid Javid told the BBC's Marr on Sunday that Johnson "will obey the law," but reiterated that the UK will leave the EU on October 31.

Johnson "will absolutely not ask for an extension" during an October council meeting with the EU, Javid said, but instead will "try to strike a deal."

On Sunday France's Foreign Minister Jean-Yves Le Drian ruled out an extension to Brexit beyond October 31 in the "current state of things," during an interview with the radio station, Europe1.

"They say they want to propose other alternative solutions and arrangements... but we haven't seen them. So for us, it is no. We are not going to restart every three months."

He added that Britain must "take control" of their "disturbing" situation.

"The British must say to us what they want... we don't want them to leave the European Union, they have decided to do it, so tell us dear British friends, what do you want to do?"

On Monday Johnson plans to schedule another vote in the House of Commons to allow him to hold an election, after he lost his first bid on Wednesday. Opposition parties came out united on Friday, saying they would not support it.

Rudd had been home secretary from July 2016 to April 2018 and previously served in the government as secretary of state for energy and climate change, parliamentary under secretary of state for climate change and assistant government whip.

Downing Street announced Sunday that Therese Coffey would replace Rudd as Work and Pensions Secretary, according to Britain's Press Association (PA) news agency.