Liberal Democrat Jo Swinson made a “genuine mistake” by accidentally voting for Jeremy Corbyn’s Brexit vision.
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.
Her 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.
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.