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.
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.