The Liberal Democrats are set to officially back revoking article 50 in an attempt to position themselves as the most pro-EU political party.

The move would effectively sever the chances of an alliance with Labour at a forthcoming general election.

The Lib Dem leader, Jo Swinson, said she would support the cancellation of Brexit, and the party expected to adopt this policy and write it into its election manifesto.

“I relish the chance to take the fight to Boris Johnson in an election and I’m confident we’d make significant gains,” said Swinson, who was elected as the party’s first female leader in July.

“Whenever the election comes, our position is clear and unequivocal. A majority Liberal Democrat government would not renegotiate Brexit, we would cancel it by revoking article 50 and remaining in the European Union.”

She will take the proposal to a vote at the party’s autumn conference in Bournemouth, which starts on Saturday.

Senior party figures have been toying for months with backing the revocation of article 50, but up to now only as an alternative to a no-deal Brexit. A source said the shift in policy was to back cancelling Brexit “in all circumstances”.

It is understood that under the proposal the party would back immediately revoking article 50 in an election campaign. Should the party not win the election, it would then go back to supporting a second referendum and campaigning to remain.

The Lib Dems have been buoyed in recent weeks by the defections of the former Tories Phillip Lee and Sarah Wollaston and the ex-Labour MPs Luciana Berger and Angela Smith. The former shadow frontbencher Chuka Umunna joined the party in June and the total number of Lib Dem MPs now stands at 17. The party has 120,000 members, its highest number ever.

At a potential election the party expects to take seats in remain-voting areas that have Labour and Tory incumbents, in line with its success at the European elections where it increased its number of seats from one to 16.

Sources in the Lib Dems said it was time for the party to set itself apart from Labour’s “renegotiation” position on Brexit and make itself an unequivocal remain option for voters.

The move effectively closes down pre-emptive conversations around an electoral pact with Labour. Swinson angered many within Labour just a month into her new role when she criticised Jeremy Corbyn’s suggestion that he should become a caretaker prime minister to stop a no-deal Brexit.

Instead she said someone like Ken Clarke, who lost the Tory whip last week after he voted against the government, or the former deputy Labour leader Harriet Harman, would be better suited to the role. The shadow international trade secretary, Barry Gardiner, described her dismissal of Corbyn as “extremely petulant”.

Umunna said the decision to campaign to cancel Brexit made the Lib Dems the UK’s “biggest and strongest” remain party. “The Liberal Democrats are the number one choice for people at the next general election who are opposed to Brexit. Our position is clear and unambiguous. A majority Liberal Democrat government would not renegotiate Brexit, we would stop Brexit,” he said.

“Ploughing headlong into a no-deal Brexit will be a huge, reckless gamble with people’s jobs and livelihoods. Jeremy Corbyn is a lifelong Brexiter and will be intensely relaxed if Brexit happens – indeed, he has spent years campaigning for it,” Umunna added.

“By contrast, the Liberal Democrats under Jo Swinson’s leadership are the UK’s biggest and strongest remain party. We are determined to do all we can not only to stop a no-deal Brexit but to stop Brexit altogether.”

The party has had a meandering journey to its now staunchly pro-EU position. In 2016 the then leader, Tim Farron, faced criticism from Sir Vince Cable, who went on to become his successor, for attempting to get the party to back a second referendum. Cable said the plan raised too many questions and would not be a “panacea to anything”.

Cable shifted to a more pro-remain position during his two-year leadership of the party, eventually making support of a public vote his party’s stance this year.

However, he bitterly resisted a challenge from Umunna, then with Change UK, to back revoking article 50. He said it would be “unsatisfactory” and not the “best outcome” and the party should instead put its energies into trying to secure a second referendum.

Swinson’s change of direction is likely to be welcomed by the party’s membership, but it will be subject to scrutiny as one of 18 motions due to be debated and voted on at the conference. There may be questions about the influence of Umunna, who is now the party’s foreign affairs spokesman.

