A COURT action which could have the effect of halting the “Super Saturday” sitting of the Westminster Parliament will be heard in the Court of Session in Edinburgh later today.

Anti-Brexit campaigners have lodged a legal action in Scotland’s highest civil court in a bid to ban the Government from putting its proposed Withdrawal Agreement before Parliament.

Jo Maugham QC said he believes the agreement, due to be debated in a special parliamentary sitting tomorrow, contravenes legislation stating it is “unlawful for Her Majesty’s Government to enter into arrangements under which Northern Ireland forms part of a separate customs territory to Great Britain”.

In an important development, Maugham argues that if the Government does not give undertakings in court to follow the law, an interdict should be granted to stop Parliament discussing the deal tomorrow.

READ MORE: Brexit: Nicola Sturgeon hits out at deal 'Scotland did not vote for'

Maugham and Joanna Cherry QC, the SNP MP, have won several court cases against the UK Government over Brexit. This action is taken by Maugham himself and follows the news of the Prime Minister reaching a deal with the EU.

Reports suggest a border in the Irish Sea with differing customs arrangements for Northern Ireland than elsewhere in the UK may form part of the Withdrawal Agreement, with negotiations ongoing in Brussels.

Maugham said he expects the case to be heard today. He claimed if the court finds the proposed agreement is unlawful the Government will be obliged to request an extension to Brexit negotiations, under the terms of the Benn Act, which stipulates the Prime Minister must ask the EU for a delay if Parliament does not agree a deal by tomorrow.

Maugham tweeted: “We believe the Government’s proposed Withdrawal Agreement is contrary to section 55 of the Taxation (Cross-border Trade) Act 2018.”

That part of the act states: “It shall be unlawful for Her Majesty’s Government to enter into arrangements under which Northern Ireland forms part of a separate customs territory to Great Britain.”

Maugham added: “We do not understand how the Government might have come to negotiate a Withdrawal Agreement in terms that breach amendments tabled by its own European Research Group.

“Unless and until Section 55 is repealed by the UK Parliament it is simply not open, as a matter of law, for the United Kingdom to enter into such an agreement.

READ MORE: Kirsty Hughes: A Brexit deal has been done ... but what’s in it?

“If the proposed Withdrawal Agreement is unlawful, the Government will be obliged to request an extension as mandated by the Benn Act and in accordance with undertakings given to the Court of Session in Vince, Maugham, Cherry v Boris Johnson.”

Maugham added a further tweet: “Here we are, after three and a half years, and it looks as though Parliament will be asked to approve on Saturday a 500-plus page document which it has not seen (indeed which does not yet exist) with epochal consequences for Great Britain, Northern Ireland and the EU and in short time. What – beyond Boris Johnson’s desire to meet his self-imposed deadline – is the rush? It would be in the interests of our Parliament to have more time to consider it.”

Scottish litigation expert Craig Connal, QC, of Pinsent Masons, said: “While some may regard this as yet another judicial challenge to Brexit, this case signals a crucial legal step change in the run up to [tomorrow’s] vote in Parliament. While previous challenges were brought by declared anti-Brexiters they were focused squarely on the lawfulness of specific actions that stopped MPs debating Brexit.

“This case is different because the petitioners argue that even if Parliament supports the deal it remains unlawful because of the treatment of the Northern Irish customs territory.

“This is legally and politically complex because it could be seen as wholly illogical for MPs to vote in favour of a deal that contains a provision that renders the deal unlawful.”

A senior Government source said: “The mask has well and truly slipped, these campaigners have spent the last three years trying to frustrate Brexit and now are trying to prevent a deal.”