The comments came on another disastrous day for the prime minister, as his own brother Jo walked out of his government in protest at his leadership.
In a move which one former Tory minister described as “absolutely devastating” for the PM’s credibility, higher education minister Jo Johnson - who has previously advocated a Final Say referendum on EU membership - said he had found it impossible to reconcile his loyalty to his brother with the national interest
Speaking during a visit to Wakefield, West Yorkshire, Boris Johnson paid tribute to his younger brother’s service in government, but made clear he was sticking to his pledge to take Britain out of the EU on 31 October come what may.
Asked if he could promise not not to ask for an extension to Brexit negotiations, as a bill going through parliament requires him to, the prime minister replied: “Yes, I can. I would rather be dead in a ditch.”
But challenged on whether he would therefore resign if the bill becomes law, as it is expected to on Monday, he simply restated his objection to an extension.
“It costs £1 billion a month, it achieves nothing," he said. "What on Earth is the point of further delay? I think it’s totally, totally pointless.”
He repeated his call for Labour leader Jeremy Corbyn to tell his MPs to vote for a snap election on 15 October, to allow time for the victor to go to Brussels for an EU summit two days later with a mandate from voters to negotiate either a deal or an extension to talks.
The prime minister's comments came in a sometimes shambolic speech at a police training academy, where he struggled to remember the formal words of an arrest caution and was forced to cut his remarks short when a female officer had to sit down after waiting in the warm sun for his delayed arrival.
There was anger at his use of a line-up of rows of uniformed cadets as a backdrop for highly partisan remarks, with Wakefield MP Mary Creagh saying she was "very unhappy" about cadets being used as "wallpaper for Johnson’s party political stunt".
"Keeps them waiting in the sun for an hour. Carries on spouting when a recruit feels faint and sits down," said Ms Creagh. "We saw today how Johnson treats the police."
Hero policeman Charlie Guenigault, who was injured in the London Bridge terror attack, said that the event sent "the wrong message" by giving the impression that officers were displaying their political allegiance.
Jo Johnson’s resignation comes just six weeks since he faced fierce criticism for returning to the government despite having resigned last year to campaign for a second EU referendum.
In a tweet announcing his decision to leave the government and stand down as MP for Orpington, he said it was impossible to reconcile “family loyalty and the national interest”, adding: “It’s an unresolvable tension and time for others to take on my roles as MP and minister #overandout “
The prime minister paid tribute to his brother as “a fantastic guy and a brilliant minister” who had supported his domestic agenda, but added: “Jo doesn’t agree with me about the EU. It’s an issue which obviously divides families and divides everybody.
"But what I think Jo would agree is that we need to get on and sort this thing out."
It is understood that Jo Johnson informed the prime minister of his plan to resign in a phone call on Wednesday evening.
One Westminster observer immediately summed up his decision to walk out as sending the message: “I’m resigning to spend less time with my family.”
Others pointed back to an interview Boris Johnson gave, a few years ago, in which he insisted he and his brother would never suffer a family schism like David and Ed Miliband.
“We don’t do things that way, that’s a very left-wing thing,” the elder Johnson said.
“Only a socialist could do that to his brother, only a socialist could regard familial ties as being so trivial as to shaft his own brother.”
It appeared the final straw for Jo Johnson was his brother’s decision to “purge” 21 moderate Conservatives from the party for their rebellion to block a no-deal Brexit.
Among the exiled MPs were colleagues, such as Justine Greening, with whom he had – before the summer – been working to try to avert a crash-out from the EU.
One of the 21, former defence minister Guto Bebb, told The Independent that the resignation was "absolutely devastating to the credibility of Boris Johnson".
"In effect, Jo is saying he has wrestled with the choice between loyalty to his brother or loyalty to the national interest and has concluded that loyalty to the national interest has to come first," said Mr Bebb.
"This is Jo Johnson - who is a decent, honourable person - saying quite categorically that his brother is not acting in the national interest."
Mr Bebb said he expected opposition parties to exploit the schism between the brothers in the same way that Conservatives did the rivalry between the Milibands, in a way which would be "very damaging" to the Tory cause in the expected election.
But another former minister told The Independent: "It's not the same. Ed Miliband was challenging his brother. Jo is vacating the scene. His decision to leave parliament makes clear he is not doing this to damage his brother."
And former justice secretary David Gauke, who had the whip removed after rebelling on Tuesday, said: "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."
Shadow education secretary Angela Rayner said: “Boris Johnson poses such a threat that even his own brother doesn’t trust him.”
The resignation capped a horror 24 hours for the new prime minister, who has yet to win a Commons vote – while suffering multiple defats at the hands of MPs.
The Commons refused to grant Mr Johnson a snap general election, less than two hours after passing a bill designed to block a no-deal Brexit on 31 October.
An election could yet be granted for his chosen date of 15 October if the bill becomes law by Tuesday, but Mr Corbyn is facing a growing Labour revolt to delay it further.
When he first resigned, in November 2018, Jo Johnson branded Theresa May’s negotiations as a “failure of British statecraft on a scale unseen since the Suez crisis”.
Warning Britain stood “on the brink of the greatest crisis since the Second World War”, the younger Johnson added: “The democratic thing to do is to give the public the Final Say.”
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