The battle against Brexit has not ended with Boris Johnson’s emphatic general election victory, Liberal Democrat interim co-leader Sir Ed Davey has said.

While accepting that the prime minister now has the votes in the Commons to get his withdrawal agreement through by 31 January, Sir Ed insisted that this will not mean Brexit is “done”.

Johnson’s plans could still “implode” over the course of the coming year as the inconsistencies in his election promises come under strain in the struggle to get a trade agreement with Brussels by the extraordinarily tight deadline of December 2020, he said.

And he said that the threat which Brexit poses to the future position of Scotland and Northern Ireland in the United Kingdom will give the lie to Johnson’s “One Nation” rhetoric, declaring: “We are the unionists now – the patriotic party who support our union.”

Sir Ed, who made an unsuccessful bid for the Lib Dem leadership earlier this year and took the position of co-leader alongside party president Sal Brinton after Jo Swinson lost her seat, refused to say whether he will put himself forward when the contest for her successor is launched – possibly several months into 2020.

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Speaking to The Independent just two days after a disastrous election which saw the Lib Dems’ representation in the Commons slashed from 21 to just 11, he insisted his party had not been thrown into disarray by what he acknowledged was a “tough” outcome.

A detailed review of the election campaign, taking in tactics, seat targeting and messaging, will be launched by the party’s federal board in the new year, but he had detected no clamour for a fundamental rethink of principles or approach of the kind seen within the Labour Party, which he described as “falling apart”.

While Labour squabbles over the succession to Jeremy Corbyn, Lib Dems aim to be on the front foot offering opposition to the Johnson government in parliament over Brexit and the climate emergency, he said.

And he made clear that he views left-wing efforts to install a new leader in Corbyn’s mould – as well as consternation among traditional Tories over the threat which Mr Johnson’s Brexit plans pose to the union and business – as an opportunity for the kind of reshaping of British politics which the Lib Dems have long dreamt of.

“I think there is still a chance the realignment of politics will happen and it will be based around the liberal progressive centre,” said Sir Ed.

“Under severe pressure in the last parliament, the Conservative and Labour parties didn’t fall apart, although they both lost a number of MPs. Let’s see what happens to the Labour Party now.”

Sal Brinton and Ed Davey have taken on the co-leadership of the Liberal Democrats on an interim basis (PA)

If Labour does not learn the lessons from the rejection of Corbyn’s left-wing agenda by the electorate, Lib Dems will be ready to offer “that centre-left reformist party which people who believe in social justice and public service and internationalist politics can feel comfortable in joining”, he said.

And he said that Johnson’s willingness to risk economic prosperity and the future of the UK for the sake of Brexit meant Lib Dems can hope to attract more of the traditional Tory voters who took them close to taking seats like Esher and Walton on Thursday.

“Given what Johnson is about to inflict on our country, there are still lots of people in commerce, in business, in finance in the Tory heartlands who can’t believe the Tory Party is now anti-business, anti-free trade, anti-market,” he said. “A lot of them will want to look to us.”

Jo Swinson lost her East Dunbartonshire seat on Thursday (Reuters)

He dismissed Mr Johnson’s promise to pursue a “One Nation” agenda at the head of a majority Tory government.

“I don’t believe anything our prime minister says,” said Sir Ed. “I wish that wasn’t the case. I have respected and felt able to believe prime ministers from other parties throughout my life, but I can’t believe anything this prime minister says.

“He talks about One Nation, but he has a deal which puts a border down the Irish Sea and he is pursuing policies which alienate Scottish people. If it is one nation Toryism to put the Union of the United Kingdom under threat, then we are in trouble.

“I want to keep the UK together, but Brexit puts this amazing country in peril, and that’s what I fear more than anything.”

Sir Ed – energy secretary in the coalition government of 2010-15 – said that as 2020 becomes mired in arguments over future trade and security relations with Europe, voters will quickly come to realise that Mr Johnson’s central election promise to “get Brexit done” was hollow.

“Getting Brexit done isn’t about the withdrawal agreement, it’s about all the many other things which are yet to be done,” he said. “I think he is not going to be able to deliver on his promise.

“Brexit could still implode. The prime minister has made a whole load of promises that are inconsistent, incoherent and probably undeliverable.”

Rejecting Mr Johnson’s attack on parliament for supposedly imposing “gridlock” on the Brexit process, he said: “It’s completely legitimate for a party like the Lib Dems to point out the complexities and contradictions involved.

“We want to make the case that Brexit is still going to be bad for the country, even if he has got a mandate to proceed with it. Our job is to speak for the people who voted for us and for the 48 per cent who voted to remain in the EU. Fifty-two per cent plus of those who voted on Thursday voted for parties which didn’t want Johnson’s Brexit to happen.”

Despite Thursday’s comprehensive dashing of Lib Dem dreams of significantly increasing their numbers on the Commons green benches – or even installing Ms Swinson in Downing Street – Sir Ed was bullish about the party’s future.

“My job at the moment is to get the party into the position where it can do its constitutional job to take forward the policies it stood on, and to be the opposition to this right-wing nationalist government,” he said. “We have got a Labour Party falling apart, divided, with its leading figures attacking each other. The country needs an opposition to stand up to the government and hold it to account. That’s what we will do.”

Sir Ed said he had not yet spoken to Ms Swinson about her plans, but would like to have her back in the political frontline “as soon as possible”.

He declined to join the chorus of criticism aimed at the former leader for her decision, viewed as undemocratic by some voters, to commit the party to revoking Brexit if it won office, saying that while he has voiced his opinions of the conduct of the Lib Dem campaign privately to the federal board, it would be wrong for him to “second guess” its judgement.

Instead, he pointed the finger of blame for the party’s underperformance at Jeremy Corbyn, arguing that Lib Dems had ended up as “collateral damage” of the electorate’s dislike and distrust of the Labour leader.

Voters who might have supported the Lib Dem agenda were scared off by the thought that they might inadvertently put Corbyn into 10 Downing Street, he said.

“We underestimated the depth of the opposition to Jeremy Corbyn,” said Sir Ed. “I don’t think it was down to Brexit. We got washed up in a wider rejection of Corbyn and Labour.