14:42

The court of session has finished hearing submissions on the latest legal challenge by anti-Brexit campaigner Jolyon Maugham QC, and Lord Pentland has said that he will give his decision by the end of the day.

Scotland’s highest court heard that Boris Johnson’s new withdrawal agreement to leave the European Union involves a “clear and unequivocal breach” of national law.

The basis of the challenge is that the newly agreed deal with the EU contravenes legislation – originally amended by hardline Brexiters to stymie the backstop arrangement – that prevents Northern Ireland forming part of a separate customs territory.

Sitting before Pentland, the court of session in Edinburgh heard from Aidan O’Neill QC that the provisions of the new withdrawal agreement make it plain that Northern Ireland would also form part of a separate customs territory, that of the European Union, and that this breaches section 55 of the Taxation (Cross-border Trade) Act 2018, which states that 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”.

Noting that section 55 – the result of an amendment sponsored by the hardline European Research Group - “intended to tie the government’s hands”, O’Neill added that whether Northern Ireland forms part of a separate customs territory after Brexit “is a question of objective law” and that it “can’t be plainer” that the UK government has acted unlawfully.

But Gerry Moynihan QC for the UK government argued that the petition was beyond the competency of the court, saying it was inviting the court to “inhibit” consideration of the agreement. “This is a manifest attempt to interfere with proceedings in parliament,” he told Pentland. He added that a substantial part of Northern Ireland’s trade would remain part of the UK’s customs territory and therefore section 55 would be complied with.

Moynihan also presented a letter from the Speaker’s counsel which warned that Maugham’s petition was asking for action that would “inevitably involve interference on proceedings in parliament and as a breach of the separation of powers”.

The hearing, which concluded just before 1pm, involved testy exchanges between Pentland and O’Neill, as the judge attempted to clarify whether the court was in effect being asked to prevent parliament debating the deal on Saturday.

O’Neill said that he was asking the court to clarify the law for parliament, and that it would be parliament’s decision whether to then repeal section 55 in order to retrospectively validate the agreement.