The purge provoked a fierce backlash among Conservative centrists. On Wednesday, a group of almost 100 Tory MPs said the PM’s order to expel moderates was “wrong in principle and bad practical politics”.

In a tweet announcing his decision, Jo Johnson, who has been an MP for nine years and is a business minister said “in recent weeks I’ve been torn between family loyalty and the national interest”.

“It’s an unresolvable tension and time for others to take on my roles as MP and minister,” he said, adding “over and out”.

While he didn’t explain further, he was reportedly furious at the purge of so many of his colleagues whose views he shared.

Labour MEP Theresa Griffin joked that it was “the first example of a politician stepping down to spend less time with their family”.

Conservative MP Mark Garnier said it was “so sad” to see Jo Johnson leave.

“Having worked with him as a minister I known him as an outstanding, hard working and clever individual,” Garnier said.

A Number 10 spokesman said Jo Johnson had been “a brilliant, talented minister and a fantastic MP”, and the prime minister “as both a politician and brother understands this will not have been an easy matter for Jo”.

An unnamed MP told the BBC that Jo Johnson had “killed” his brother by resigning at such a pivotal moment.

The brothers have long differed over Brexit. In November 2018, Jo Johnson criticised his brother’s “false prospectus” and “fantasy promises” in the Brexit referendum.

Responding to the news, sister Rachel Johnson, a writer and journalist who joined the Liberal Democrats in 2017 in protest against Brexit, said her family “avoids the topic of Brexit especially at meals as we don’t want to gang up on the PM!”.

A general election is imminent in the UK after the Boris Johnson government lost its majority in Parliament.

However, opposition parties are blocking the Prime Minister’s push for a snap election, saying they first want to make sure a law passes and receives royal assent to prevent the government choosing a no-deal Brexit on October 31.

Boris Johnson’s purge of disloyal MPs is not unprecedented – in 1993 John Major withdrew the government whip from nine MPs who had refused to support him on implementing an EU treaty into British law.

But in that case the whip was later restored – while this time Downing Street is reportedly determined to make sure all current rebels lose their seats in the upcoming election, replaced by loyal newcomers who would not block a no-deal Brexit.

But the purge includes some very senior figures.

Nicholas Soames, a 37-year House of Commons veteran who is also Winston Churchill’s grandson, was among the Tory rebels who criticised Boris Johnson for his recent tactics in parliament on Wednesday.

“I am truly very sad that it should end in this way,” he said, pointing out that he had always voted for the government’s Brexit deal to pass the Parliament.

“Which is more than can be said for my right hon. Friend the Prime Minister, the Leader of the House and other members of the Cabinet whose serial disloyalty has been such an inspiration to so many of us,” he added.

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