Senior lawyers, business leaders and academics urged the government to give MPs the power to block an ‘unpatriotic Brexit’ if the Prime Minister, Theresa May, does not strike a deal that is in the national interest.

A letter in The Times newspaper, organised by Mark Stephens, a media law partner at London firm Howard Kennedy, came as peers began to debate the controversial ‘Brexit Bill’.

It said: ‘We believe Parliament should amend the Article 50 notification bill to ensure that it can determine what should be done if negotiations break down.

‘Parliament’s vote on any emerging settlement must also permit, if the terms are not in the national interest, amendment or extension of the negotiations, and to allow the country the option of an alternative relationship with the EU, including the possibility of membership.’

Signatories included Baroness Kennedy QC, Ben Emmerson QC, of Matrix Chambers, Alexander Layton QC, of 20 Essex Street, Peter Montegriffo QC, John Vater QC, of Harcourt Chambers, Tim Ward QC and Jon Turner QC, of Monckton Chambers, and Jolyon Maugham QC, of Devereux Chambers.

It followed an opinion from three of the UK’s most senior EU law experts – Sir David Edward QC, Sir Francis Jacobs QC, and Sir Jeremy Lever QC, commissioned by law firm Bindmans. Dubbed the ‘three knights opinion’, it suggested that the EU (Notification of Withdrawal) Bill does not authorise Brexit and that a further Act of Parliament would be required if it is to occur in a way that is lawful.

Parliament passed the Brexit Bill unamended on 13 March.