The Liberal Democrats have pledged to cancel Brexit if they come to power at the next general election.
Members voted for the new policy at their party conference in Bournemouth by an overwhelming majority.
Previously, the party has backed another referendum or "People's Vote", saying they would campaign to Remain.
After the vote, their leader Jo Swinson, said: "We will do all we can to fight for our place in Europe, and to stop Brexit altogether."
The commitment only comes into force if the party wins the election as a majority government.
Ms Swinson also confirmed that before an election is called, the Lib Dems would continue to work with other opposition parties to campaign for a further referendum, and to prevent a "dangerous" no-deal Brexit.
She told BBC Radio 4's Today programme: "We still want to have a People's Vote. We've been arguing for that for the last three-and-a-half years - [to put] the Brexit deal to the public in a referendum.
"[But] when we have an election, if we haven't had a People's Vote, people will be looking to resolve the issue of Brexit, and there are so many people in this country who are so sick of hearing about it.
"They want to get on with their lives and want the government to get on with making their lives better."
"We are going to fight to stay in the EU."— BBC Radio 4 Today (@BBCr4today) September 16, 2019
Lib Dems' leader @joswinson is "determined" for the party to stop Brexit but won't say whether it'll become a 'rejoin the EU' party if the UK leaves #r4today https://t.co/i2LffYHEUd pic.twitter.com/c8rTcmZuVI
In his first speech to conference as a Lib Dem MP, Chuka Umunna - who left Labour over its Brexit stance - said it would give the party a "clear, unequivocal position".
He said: "This [policy] will stop this national embarrassment and enable us to focus on the things that really matter."
But fellow Lib Dem MP Norman Lamb said the policy saw his party "playing with fire".
He told the Today programme that the polarisation between Leave and Remain was "incredibly dangerous", adding: "If we take this to the very limit in a situation where one side is vanquished entirely, I think there's a real danger that we break the social contract in our country.
"And I think that we all have a responsibility of reuniting the country in a common endeavour."
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.
Home Secretary Priti Patel said the "entire machinery of government" was focused on securing that deal.
The PM is due to meet European Commission president Jean-Claude Juncker in Luxembourg later, as negotiations aimed at securing a deal continue.
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" than ask for a delay, and the UK will leave the EU at the end of next month "whatever happens".
The Lib Dems' motion said that if the party became the government at the next general election, it would revoke Article 50 - the law that ensures the UK leaves the EU.
Earlier, 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.
"Everybody can see we are stuck, that Brexit is in a mess. There needs to be a way out of that."
Moving the motion in Bournemouth, Sir Vince Cable 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 all important 'if'
Analysis by BBC political correspondent Jonathan Blake
In Brexit terms, revoking Article 50 could be considered the nuclear option, stopping dead the process of leaving the EU.
It's just what the Liberal Democrats want and now they've adopted a policy to do exactly that - if they win a general election.
But the most important word in the last sentence is "if".
If they don't find themselves in government they will, we can assume, revert to campaigning for a further referendum as the best way to reverse the result of the last one.
So, this policy allows the party to send a message to voters that they are as opposed to Brexit as it's possible to be.
But it's not without risk for a party with the word "democrat" in its name to promise to overturn the result of a referendum without putting that question to the electorate again.
The Lib Dems are enjoying a resurgence on the back of their 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.