The former Liberal Democrat cabinet minister Ed Davey launched his bid for the party leadership on Thursday, pledging to fight a no-deal Brexit by working with remainers in parliament to make revoking article 50 the legal default if no agreement on leaving the EU is reached by October.

Davey, the former secretary of state for energy and climate change, said he would make stopping Brexit the cornerstone of his leadership, but also said the party must broaden its appeal and that he would do so by focusing on the environment.

The MP, who regained his Kingston and Surbiton seat at the 2017 general election, is likely to be in a two-horse race with the party’s deputy leader, Jo Swinson, to replace Vince Cable when he steps down in mid-July. Swinson is expected to announce her candidacy later this week.

Davey said he would not wait for the results of the leadership contest to begin work with other remain MPs on finding a legal route to stopping no-deal – a prospect boosted by comments made by the Speaker, John Bercow, who made it clear on Wednesday he did not believe no-deal could occur without MPs’ consent.

“We want to pass a law that if you got to the cliff edge that the default would be revocation, not crashing out,” Davey said. “People who realise no-deal is a disaster ought to vote for that legislation – I include people in the current cabinet.”

The MP said he saw another radical route to stopping no-deal – a vote of no confidence in the government and the formation of a unity government to deliver a second referendum. “I would not go for a general election immediately – the Queen has to first determine if anyone else can form a government,” he said.

“She would probably go to Jeremy Corbyn first but I can see MPs across parties, like Tom Watson, Hilary Benn, the SNP, Plaid [Cymru] and Tory remainers, possibly even Tory cabinet ministers, could make it clear publicly that they would form a temporary government with the sole purpose to allow a people’s vote. In those circumstances, we should do that. We have to stop no-deal.”

Davey said he could not envisage any future formal coalitions with the Conservatives or Labour in their current forms, because of the Tory position on delivering Brexit and because it would be “deeply unattractive” for the Lib Dems to enter a formal coalition with Jeremy Corbyn. “He has no credibility on this issue,” Davey said.

That does not mean the Lib Dems are afraid of entering government again, he said: “I just look at the current bunch and I don’t want to go anywhere near them.”

Q&A How do Liberal Democrat leadership elections work? Show The Liberal Democrat leader is chosen by party members in a postal ballot. To become a candidate, an MP must secure nominations from at least 10% of the Liberal Democrat MPs in parliament, and from at least 200 members from more than 20 local parties. The ballot is usually carried out using the single transferable vote system. Voters rank their choice of leader in preference. After the first preference votes are counted, if no candidate has over 50% of the vote, then the second preference votes of those who cast their vote for the least popular candidate are reallocated, and so on, until somebody has a majority. In the 2019 race however, only Jo Swinson and Ed Davey are running, so the ballot will be a straight head-to-head contest.

The leadership contenders are likely to face intense questioning from party members who saw botched attempts to form pacts with other remain parties such as Change UK and the Greens fail.

Davey said he felt the successes for the party in the most recent elections put it “in a very different place” now in those discussions, hinting that he could prefer defections from Change UK MPs rather than pacts.

“I can understand why a year ago they thought we were not doing well,” he said. “Change UK may have had that view, but they have been proven totally wrong. I would like to see now big players from the major parties having the courage to leave, but not to come to Change but to come to us.”

Swinson has previously said the Lib Dems must “own the failures” of coalition, citing some deep cuts to public services, the introduction of the hostile environment and the bedroom tax.

However, Davey said he will “never say the coalition was a bad thing” and that the Lib Dems’ difficult reputation in coalition had only been down to their PR.

“We played the politics wrong, but we did look after the people who needed looking after and we stopped the Tories cutting far deeper and more dramatically,” he said. “We got the blame for the bad things and no credit for the good things.”

Davey intends to draw on his former climate brief to give the Lib Dems another key policy to champion.

“Why we haven’t prospered for quite a period, in my view, is that people forgot what the Lib Dems stand for – apart from stopping Brexit. We’ve had success on the back of ‘events, dear boy’, like the Iraq war and Brexit, but that comes and goes – and then what?”

Davey said the climate crisis was “the issue” of the next generation. “The science is showing it far worse than we thought when I was in government,” he said. He intends to launch a policy putting tighter restrictions on investment in fossil fuels in the City of London.