These lawyers told us how they are trying to block the government from triggering Article 50

One of the most important legal cases in Britain's constitutional history will get underway at the Supreme Court on Thursday.

Three of the most senior judges in the UK will rule whether the government is legally required to pass an act of parliament before triggering Article 50 and formally starting Britain's divorce from the European Union.

Mischon de Reya, a law firm based in central London, will represent one of the claimants, businesswoman Gina Miller, who Business Insider interviewed in August.

Miller's case is based on the argument that UK government cannot use royal prerogative to take Britain out of the European Union.

The case will not only go a long way to determining how and when Britain leaves the 28-nation bloc but will re-shape the country's constitutional landscape. "It is a constitutional blockbuster," barrister Jack Williams told BI.

Williams, along with fellow lawyers Gerry Facenna QC, who has acted on behalf of the UK in numerous cases in the European Court of Justice, and Anneli Howard, outlined the key arguments which will be raised in Miller's case.

These are the basic arguments (paraphrased):

• Has a constitutionally-valid decision been made for the UK to withdraw from the EU and formally notify its withdrawal under Article 50?
• Can the Government notify the decision to withdraw using royal prerogative alone or must it secure Parliamentary approval?

In particular:

• Hasn't the government's power to use royal prerogative been removed by past legislation, such as the European Communities Act (1972), the Bill of Rights (1689), Acts of the Union and creation of devolution settlements for Northern Ireland, Scotland, and Wales?
• Does royal prerogative extend to allowing the government to remove fundamental citizenship rights derived from EU law?
• Would exercising royal prerogative be abusive by undermining Parliamentary Sovereignty, the proper role of the government as outlined by the constitution, and/or the rule of law?

It may sound complicated. But the premise of the case is pretty simple: it is not lawful for Theresa May's government to use royal prerogative to trigger Article 50. You can read the claimant's skeleton argument in full at Mischon de Reya's website.

The defendant in the case will be Secretary of State for Exiting the European Union, David Davis, who will be represented by the government's legal department.

If the court rules in the claimant's favour, the majority of MPs would need to back an act of parliament before May can trigger Article 50. In theory, this means MPs could delay the process if they were unhappy with the the prime minister's negotiating position.

The dates of hearing will be the October 13, 17, and 18, with a verdict expected in early-November.