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Boris Johnson's own brother quit politics today saying he was “torn between family loyalty and the national interest”.

In his resignation statement, Jo Johnson said he could not serve with the “unresolvable tension” caused by his brother’s threats to crash out of the European Union without a deal.

It came just hours before the Prime Minister was due to make a speech in Yorkshire seen as a bid to launch the next general election campaign.

Quitting as both a senior minister for universities and science who attended Cabinet and as Tory MP for Orpington, Mr Johnson announced his decision in a tweet.

“It’s been an honour to represent Orpington for nine years & to serve as a minister under three PMs,” he said. “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.” He signed off with the hashtag #overandout.

The family rift blew open as the Conservatives were being torn apart by the Prime Minister’s decision to strip the Tory whip from 21 MPs who rebelled to stop a no-deal, including Winston Churchill’s grandson Sir Nicholas Soames.

Former justice secretary David Gauke, another of the sacked rebels, commented: “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.”

To some MPs the family division echoed the split between David and Ed Miliband, rising stars who served in Gordon Brown’s Cabinet but fell out when the latter stood for Labour leader. Ed’s leadership was blighted by the public perception that he stabbed his own brother in the back.

Asked in 2013 whether he and his brother resembled the Milibands, Boris Johnson said: “Absolutely not. We don’t do things that way, that’s a very Left-wing thing ... only a socialist could regard familial ties as being so trivial as to shaft his own brother.”

Former Financial Times journalist Jo Johnson was a passionate Remainer at the 2016 referendum, like sister Rachel who stood as a Change UK candidate in the European elections this year.

A No 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 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.”

Government sources said Jo Johnson broke his news in a phone call to his brother last night.

The Prime Minister looked impassive as he stood outside No 10 for the arrival of Israeli Prime Minister Benjamin Netanyahu for talks shortly after his brother’s announcement.

MP Margot James, who also lost the Tory whip, told Jo Johnson in a tweet: “If I thought my decision [to vote against the Government this week] had been difficult it was nothing compared to yours. I am utterly humbled by your action and statement today.”

Mr Johnson will be in demand as a journalist, City expert or in the higher education field. His departure may put pressure on other ministers uncomfortable about a no-deal Brexit to consider their positions.

The drama erupted as an autumn general election moved closer. Opposition party leaders held phone talks at Westminster to hammer out dates running from “late October” to early December — but ruled out the October 15 polling day being demanded by the Prime Minister.

Shadow chancellor John McDonnell said the issue was whether to “go long” or “go short” in terms of how soon the country votes.

Mr Johnson planned to come out fighting this evening by accusing Jeremy Corbyn of “a cowardly insult to democracy” if he refuses a snap election. Speaking in Yorkshire, he will say the election choice would be framed as Brexit on October 31 with him as PM, or “more delay, more dither and accepting whatever terms Brussels imposes over our nation” under Labour.

But opposition parties insisted they will call the shots after a dramatic week of parliamentary defeats for Mr Johnson. Labour sources said leader Mr Corbyn could back an election in late October or November. Mr McDonnell said: “We are going to get one ... But we are going to control the timing.”

Liberal Democrat leader Jo Swinson, interviewed in tonight’s Standard, called for a delay until November to make sure the Prime Minister obeys legislation being passed by Parliament to force a Brexit extension in the absence of a withdrawal deal. “I think we need to make sure that legislation is implemented,” she said, adding: “I suppose November would be possible.”

In key developments: