The Supreme Court today threw a further hurdle in the way of Theresa May’s hopes of a smooth Brexit, when it ruled the Scottish and Welsh governments can intervene.

Edinburgh and Cardiff will be allowed to make their separate cases to the court for the right to have a say over the triggering of the Article 50 notice period.

The decision raises the possibility – albeit thought to be slim – of the Supreme Court agreeing with the SNP that the Scottish Parliament should have a veto over the Brexit strategy.

That would plunge the United Kingdom into a full-blown constitutional crisis, as well as potentially sink the Prime Minister’s exit timetable.

The decision is a big victory for Nicola Sturgeon, the SNP’s first minister, who has insisted Scotland’s voice must be heard after the country voted decisively to Remain.

The Government's appeal against the High Court ruling that MPs must give their consent to the invoking of Article 50 will be heard, over four days, from December 5.

The decision is expected at the start of January, after which – if it loses – the Government will introduce a short three-line Bill to try to keep Brexit on track.

Ms May has said she will trigger Article 50 by the end of March, beginning two years of formal exit talks expected to conclude with Britain leaving the EU in spring 2019.