British Prime Minister Boris Johnson received a fresh blow Saturday when senior minister Amber Rudd quit her work and pensions post in protest at his handling of the Brexit crisis.

Her resignation caps a miserable week for Johnson as he tries to steer his splintered country through its biggest political crisis since World War II.

Rudd was a moderate member of former prime minister Theresa May's government whose endorsement Johnson coveted during his successful UK leadership challenge in July.

But she said Saturday that she could no longer endorse Johnson's approach to negotiations with Brussels - or handling of domestic politics.

"I have resigned from Cabinet and surrendered the Conservative Whip," Rudd tweeted.

"I cannot stand by as good, loyal moderate Conservatives are expelled," she said referring to Johnson's decision to expel 21 MPs from the Conservative party for voting against the government on Brexit.

Rudd said in her resignation letter that she felt uneasy about Johnson's commitment to take Britain out of the EU on October 31 even if the two sides fail to reach a negotiated deal.

The 56-year-old said she had once viewed Johnson's threat of a messy "no-deal" divorce as a useful negotiating tactic to take with Brussels.

"However, I no longer believe leaving with a deal is the government's main objective," she wrote in her resignation letter.

"The government is expending a lot of energy to prepare for 'no deal' but I have not seen the same level of intensity go into our talks with the European Union."

Johnson has been adamant that he will not seek a third Brexit delay this year.

He is instead seeking to hold an early general election on October 15 that could give him a mandate to take Britain out on time and at any cost.

Johnson said this week that he would "rather be dead in a ditch" than seek a Brexit extension until January that more moderate forces demand.