Liberal Democrat members have officially endorsed revoking Article 50 without a referendum if leader Jo Swinson wins power at the next general election.

In a major shift for the unashamedly pro-EU party, activists backed the inclusion of the new policy of cancelling Brexit altogether in the Lib Dems’ next manifesto for government.

The policy move comes as Sam Gyimah – a former Conservative leadership contender – defected to the Lib Dems at the party’s conference opening rally in Bournemouth on Saturday evening, in a major coup for the new leader.

Ms Swinson claimed that the new position was “very clear”, adding: “If the Liberal Democrats win a majority at the next election, if the party put into government is a stop-Brexit party, then stopping Brexit is exactly what they will get.”

She continued: “We have argued that a specific Brexit deal should be put to a People’s Vote to give clarity.”

“We will still argue for that. But if we end up at a general election then I think we need to be straight forward with people and give them an option for all this Brexit chaos to stop.”

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The new policy is designed to distinguish the Lib Dems’ Brexit message from others at Westminster, and ensure they are the most fervently Remain-supporting party at the next election, which could be as early as November.

The motion – passed overwhelmingly on Sunday – states a Liberal Democrat majority government would be “recognised as an unequivocal mandate to revoke Article 50 and for the UK to stay in the EU”.

But for Ms Swinson’s policy to be implemented, she would need to gain an election upswing never seen before in British electoral history – as it stands, the Lib Dems have 18 MPs in the House of Commons.

Speaking to the BBC’s Andrew Marr programme, Ms Swinson also vowed not to support efforts to instal Jeremy Corbyn as prime minister if an election resulted in another hung parliament.

“Boris Johnson and Jeremy Corbyn are not fit to be prime minister,” she said. “We see that day in, day out.