Boris Johnson’s premiership was seriously undermined by two major defeats in the Commons on Wednesday night but these were nothing compared with the grievous personal blow he suffered on Thursday morning when his younger brother, Jo Johnson, resigned from the government, citing a conflict “between family loyalty and the national interest”.

In a tweet just after 11am, Jo, 47, eight years younger than the Prime Minister, dropped his bombshell: “It’s been an honour to represent Orpington for 9 years & to serve as a minister under three PMs. 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.”

Later, he would only say: “It’s time to move on.”

“Disaster for Boris as Brother Quits,” was the headline in the early editions of the London Evening Standard, now edited by George Osborne, who was chancellor under David Cameron.

One report disclosed: “The PM and Jo Johnson had a long phone conversation last night about his resignation. Boris urged Jo to reconsider overnight. He didn’t. PM said nothing about it to even his most senior aides.”

Making a pitch for an election during a speech in Yorkshire, Boris described his brother as a “fantastic guy... who did a fantastic amount of work for us. Jo doesn’t agree with me about the EU. It is an issue that divides families and everybody. But I think he would agree we need to get on and sort this out.”

He explained why he was pressing for an early election: “So what I want to do now is give the country a choice. Either go forward with our plan to get a deal, take the country out on October 31 or else somebody else should be allowed to see if they can keep us in beyond October 31.”

This is merely speculation but Jo might also have had reservations about Boris splitting with his wife of 25 years, Marina Wheeler, while she has been battling cervical cancer..

Boris’s four children have not been photographed recently with their father.

Jo, who opposes a no-deal Brexit, supported Remain during the EU referendum, as did his father, Stanley, and 54-year-old sister, Rachel, who is a journalist and girl about town. The family came together during Boris’s premiership bid but Stanley will now have to walk a tightrope between his sons.

At a charity function on Wednesday, Rachel quipped that “the family avoids the topic of Brexit especially at meals as we don’t want to gang up on the PM!”

Boris’s formal reaction was delivered by his spokesman at 10 Downing Street: “The Prime Minister would like to thank Jo Johnson for his service. He has been a brilliant, talented minister and a fantastic MP. The PM, as both a politician and brother, understands this will not have been an easy matter for Jo. The constituents of Orpington could not have asked for a better representative.”

There has been plenty of drama at Westminster in the last couple of days but no one was quite prepared for British politics venturing into Mukesh Ambani vs Anil Ambani territory, which it did with Jo announcing he would be quitting as universities and science minister and also stepping down as an MP at the next election.

Jo’s resignation was seized on by Labour but perhaps the most analytical assessment came from the BBC’s assistant political editor, Norman Smith, who said: “Every day something utterly astonishing happens here. This is one of the most astonishing things.

“This isn’t some minor family tiff. This is Jo Johnson trying to cut the political legs from under his brother in a very, very public way because he is saying in effect, ‘I don’t really trust my brother. I don’t have confidence in what he’s doing. I don’t think I can carry on as a Tory MP.’

“And the timing and the fact that he’s chosen to go public with it in this way is designed to basically put the skids under his brother.”

“If you just think in your own circumstances in a family, if you have a row, if you have a bitter dispute, most families you keep it within the family. You don’t go public in the way Jo Johnson has done unless you really want to create mayhem.

“Clearly Jo Johnson is deeply worried about his brother and what he’s doing as prime minister and where he is leading the country.

“And the problem for Boris Johnson is people will think if his brother doesn't trust him, if his brother is so worried about what he’s doing that in the national interest he thinks he has to stand down, then why on earth should anyone else trust him.

“This isn’t just a personal blow – and I imagine it’s deeply wounding to Boris Johnson – but politically this is a significant moment and I would say it’s very very damaging.”

Jo’s announcement could not have come at a more vulnerable time for Boris, who lost a Commons motion by 329 votes to 300, compelling him to delay Britain’s exit from the EU from October 31, 2019, to January 31, 2020. This Bill was was being debated in the Lords on Friday prior to it being given royal assent.

Boris has declared that under no circumstances would be seek an extension, thereby putting him at odds with the law. He will table a new motion on Monday, naming probably October 15 as the date for a general election.

On Wednesday, he suffered his second defeat when he failed to get a two-thirds majority under the Fixed Term Parliament Act for an election on October 15. In all, 298 Tory MPs voted for him, and there were 56 votes against. But Labour MPs abstained, which meant he was unable to get the 434 votes he needed in order to secure the general election.

Boris Johnson outside his home in London on June 13, 2019. (AP)

Monday’s motion will require a simple majority. The numbers for that vote might be tight.

On Thursday, Boris travelled to Yorkshire to make a speech, setting out the case for a “people vs parliament” election, claiming the Labour leader Jeremy Corbyn wants to “surrender” to Brussels by “begging” for a delay beyond the October 31 Brexit deadline.

But the talk all day was all about Jo’s departure.

There was a blistering comment from Sir Max Hastings, who was editor of the Daily Telegraph when Boris was on his staff: “To lose a vote of confidence to your own brother is quite a dramatic development even by the standards of the Johnson family.”

Speaking in the Commons, the Leader of the House Jacob Rees-Mogg – his sister Annunziata is a Brexit Party member of the European Parliament – was philosophical: “This is something we know about across the country – families disagree on Brexit. We all have in our families disagreement over an issue that is of fundamental importance to us all. That is why it is right to put it back to a general election so they can decide.”

Chief Secretary to the Treasury Rishi Sunak called Jo “a friend” and “fantastic guy”, but insisted the Prime Minister was “absolutely” acting in the national interest: “Every MP will have their own view of what they think is best. I know this would have been a very personal and very difficult decision for Jo, but one that was done without any malice at all.”