The party’s deputy leader was shown to have supported a Labour amendment to the Government’s Brexit motion on Wednesday night, according to the division list.

The Scottish MP’s apparent defiance of her party’s policy raised eyebrows in Westminster, but a party source said it was a “genuine mistake”.

She was the only Lib Dem MP to support the policy.

Liberal Democrats deputy leader Jo Swinson. Picture: Gareth Fuller/PA Wire

Corbyn referendum pressure after Labour’s Brexit plan defeated

Ms Swinson has been the MP for East Dunbartonshire since regaining her seat in 2017.

Labour’s amendment called for a permanent and comprehensive customs union with the EU; dynamic alignment on rights and protections; commitments on participation in EU agencies and funding programmes; “unambiguous” agreements on the detail of future security arrangements; and close alignment with the single market.

The Lib Dems tagged their own amendment on to Mr Corbyn’s proposals, calling for an extension of Article 50 to provide time for a referendum.

Mr Corbyn’s proposals were defeated by a margin of 240 votes to 323.

Lib Dems Jo Swinson voted for Corbyn’s Brexit vision