The Liberal Democrats will be the “stop Brexit party” at the next election if members agree a new policy later, their leader Jo Swinson says.
Ms Swinson wants her party’s manifesto to pledge revoke Article 50 – the law that ensures the UK leaves the EU.
Previously, the party has backed another referendum or “People’s Vote”, saying they would campaign to Remain.
Members will vote on whether to agree the new stance at the party’s conference in Bournemouth later.
Ms Swinson told the BBC’s Andrew Marr show: “If people put [the Lib Dems] into government… the stop Brexit party, then stopping Brexit is exactly what people will get.”
The government says it is trying to get a deal with the EU so it can leave on 31 October – the current deadline agreed with the EU.
MPs passed a new law earlier this month that forces Boris Johnson to ask the EU for an extension to the deadline if a deal isn’t agreed by 19 October – two days after a key EU summit.
But the prime minister has said he would rather be “dead in a ditch” that ask for a delay, and the UK will leave the EU at the end of next month “whatever happens”.
The PM is due to meet European Commission president Jean-Claude Juncker in Luxembourg this week as negotiations aimed at securing a deal continue.
‘A way out’
The motion being debated at the Lib Dem conference says: “Conference calls for Liberal Democrats to campaign to stop Brexit in a general election, with the election of a Liberal Democrat majority government to be recognised as an unequivocal mandate to revoke Article 50 and for the UK to stay in the EU.”
Ms Swinson told Andrew Marr: “Everybody can see we are stuck, that Brexit is in a mess. There needs to be a way out of that.”
She said her party had argued for another referendum on a specific deal, allowing “clarity” on what the public want.
But she added: “If we end up at a general election then I think we need to be straight forward with people and give them the option for all this Brexit chaos to stop.
“I think people want parties to be straight-forward with where they stand.
“I recognise that not everyone agrees with the Lib Dems on this [but] it is genuinely what we think is right for the country.”
Her stance was backed by her predecessor, Sir Vince Cable, during his speech at the conference on Sunday, saying there was “no better deal than no Brexit”.
Sir Vince said: “Brexit will make us poorer and risks breaking up our United Kingdom.
“We must stop it and we will.”
He added: “Jo [Swinson] is ready to steer us back into government as our new captain.
“And now, I am full of confidence and hope for our party and for our country.”
The Lib Dems are enjoying a resurgence on the back of its anti-Brexit stance.
The party currently has 18 MPs, having been boosted by a victory in the Brecon and Radnorshire by-election, along with defections from both Labour and the Conservatives over the summer.
The latest to join the ranks is former Conservative Sam Gyimah, who had the Tory whip removed earlier this month when he voted to block a no-deal Brexit.