Theresa May has been accused at the High Court of attempting to start the Brexit process by unlawfully using ancient executive powers under the royal prerogative.
In one of the most important constitutional cases in generations, a QC argued the prime minister had no legal power to trigger Article 50 of the Lisbon Treaty to leave the European Union without the prior authorisation of Parliament.
Mrs May announced at the Conservative Party conference that she intends doing so by the end of March 2017.
Lord Pannick QC attacked her decision on behalf of Gina Miller, an investment fund manager and philanthropist living in London who voted Remain in the EU referendum on June 23.
Ms Miller is the lead claimant in a historic legal action with several other applicants, including the so-called “People’s Challenge” which has the backing of thousands of supporters, all seeking to overturn Mrs May’s decision in judicial preview proceedings at the Royal Courts of Justice in London.
Lord Pannick told three judges in a packed courtroom, with other members of the public listening in two overspill courtrooms, that the case “raises an issue of fundamental constitutional importance concerning the limits of the power of the Executive”.
The QC argued Mrs May could not use royal prerogative powers to remove rights established by the European Communities Act 1972, which made EU law part of UK law, as it was for Parliament to decide whether or not to maintain those statutory rights.
He said the question of the legal limits on Executive power “arises in the context of one of the most important of our statutes which is the source of so much of the law of the land.”