Boris Johnson’s brother has quit the Government, saying he had been “torn between family loyalty and the national interest”.
Jo Johnson, who had been a senior minister attending meetings of his brother’s Cabinet, said there was an “unresolvable tension”.
He announced on Twitter he was quitting his role as Universities Minister and would stand down as MP for Orpington.
“In recent weeks I’ve been torn between family loyalty and the national interest – it’s an unresolvable tension & time for others to take on my roles as MP & Minister. #overandout”
Jo Johnson is pro-European and has previously called for a second referendum, a position that puts him at odds with the Prime Minister, who has vowed to leave the EU on October 31 with or without a Brexit deal.
It is understood he will stand down as an MP at the next election.
A Number 10 spokesman said: “The Prime Minister would like to thank Jo Johnson for his service.
“He has been a brilliant, talented minister and a fantastic MP.
“The Prime Minister, as both a politician and brother, understands this will not have been an easy matter for Jo.”
Former justice secretary David Gauke, one of those who lost the whip for rebelling against the Government this week, said Jo Johnson’s decision to resign was a “big loss”.
“Lots of MPs have had to wrestle with conflicting loyalties in recent weeks.
“None more so than Jo. This is a big loss to Parliament, the Government and the Conservative Party.”
Jo Johnson’s departmental boss Andrea Leadsom said he had been a “huge asset” to the Government.
The shock resignation came as his brother geared up for an election campaign which will focus on his uncompromising approach to Brexit.
Downing Street said the Prime Minister would set out his case directly to the public on Thursday, claiming Jeremy Corbyn wants to “surrender” to Brussels by “begging” for a delay beyond the October 31 Brexit deadline.
Labour hit back, with shadow chancellor John McDonnell comparing the Prime Minister to a toddler throwing a tantrum and arguing the Opposition was behaving like the “adult in the room”.
Opposition parties are in talks about how to respond to Mr Johnson’s call for a snap election on October 15, with concern about whether the poll should be delayed until after an extension has been secured to prevent a no-deal Brexit on October 31.
A cross-party bid to require a Brexit extension cleared the Commons on Wednesday and, after a deal struck in the early hours of Thursday, the Tories have agreed not to attempt to talk it out in the Lords.
A Number 10 spokesman said: “It is clear the only action is to go back to the people and give them the opportunity to decide what they want: Boris to go to Brussels and get a deal or leave without one on October 31, or Jeremy Corbyn arriving in Brussels with his surrender Bill begging for more delay, more dither and accepting whatever terms Brussels imposes over our nation.”
Setting the scene for a “people versus Parliament” election strategy, the spokesman said: “Boris will argue that it is now time for the people to decide after Parliament has failed them so we can resolve this once and for all.
“For Jeremy Corbyn to continue to avoid an election would be a cowardly insult to democracy.”
Mr Johnson had called for a poll to be held on October 15 but Labour and other opposition MPs refused to back the bid – which needed a two-thirds majority in the Commons – while the risk of a no-deal remained.
The Government failed to secure the support of two-thirds of MPs, with the Commons voting 298 to 56, 136 short of the number needed.
Cabinet ministers reportedly raised concerns with Mr Johnson, and the One Nation group of Conservatives called for the 21 to be reinstated.
One Nation group leader and former Cabinet minister Damian Green told the BBC: “I’m afraid it does look as though somebody has decided that the moderate, progressive wing of the Conservative Party is not wanted on voyage.”
In Brussels, frustration about the Prime Minister’s approach to seeking a Brexit deal was illustrated by a Financial Times report that chief negotiator Michel Barnier claimed there is “a state of paralysis” in talks with the UK.
The Prime Minister has repeatedly claimed progress is being made and told MPs on Tuesday that “in the last few weeks, I believe the chances of a deal have risen”.