On 19 July 2016 at the High Court, Sir Brian Leveson and Mr Justice Cranston held the opening hearing of the Brexit judicial review proceedings. These proceedings are important because the EU Referendum result in itself does not legally trigger Brexit, leaving the issue outstanding of what does.

The opening hearing was to settle some preliminary administrative details, so the full legal arguments have yet to emerge.

However, the key question is: what is the lawful way for the Government to trigger the UK's departure from the EU?

The case for the Government 

The Government has decided that the lawful process to trigger Brexit is for the Prime Minister directly to notify the European Council of the UK's intention to withdraw from the EU, under Article 50 of the Treaty of Lisbon 2007.

The Government argues that it can lawfully do so using its powers of the "royal prerogative". This is an ancient authority inherited from the monarchy for making decisions without a Parliamentary vote, among other things on foreign affairs.

The royal prerogative exists by tradition and custom, not by any officially written-down code or constitution. The Government is yet to say exactly how the royal prerogative includes the power to trigger Brexit. As John Penrose MP, for the Government, told the House of Commons on 11 July 2016: "I am sure that everyone will be aware of the debate about whether invoking Article 50 can be done through the royal prerogative… I simply remark that Government lawyers believe that it is a royal prerogative issue."

At the 19 July 2016 hearing, the Government did not detail its argument. Its representative as the Defendant is expected to be the Secretary of State for Exiting the European Union, the newly created post established following the reshuffle and currently held by David Davies.

The developing case against the Government

The claimants are challenging the Government's decision that the Prime Minister may lawfully trigger Brexit through the royal prerogative. The claimants include Mr Deir Dos Santos, said by his legal representative Dominic Chappell QC to be "just an ordinary guy", although the extent of the claimants is yet to be fully confirmed.

Their argument is that it would be unlawful for the Prime Minister directly to notify the European Council under Article 50, because lawfully Parliament must first vote its approval.

A basis of their argument is that the Referendum is not legally binding, and making Brexit a lawful reality requires new legislation that under the UK's unwritten constitution only Parliament can bring into being.

Such new legislation might cover Parliamentary approval to repeal the domestic law – the European Communities Act 1972 – that makes the UK part of the EU.

The public debate

Those bringing the case against the Government say their concern is to make sure the process of triggering Brexit is lawful. Their critics say their real aim is to frustrate the Brexit process by diverting it into Parliament for a further vote.

What is certain is that before the Referendum, the Government did not clarify the process in the event of a vote in favour for Brexit. The House of Commons Foreign Affairs Select Committee considered this in its 19 July 2016 report on "Equipping the Government for Brexit", noting that:

"Since the referendum, the extent of the Government’s lack of preparation for a potential “leave” vote has become more evident. In the light of the appointment of the new Prime Minister on 13 July, the previous Government’s confidence that basic planning for the practicalities of implementing Brexit could be undertaken at a leisurely pace after the vote now appears at best naïve and at worst negligent. The previous Government’s considered view not to instruct key Departments […] to plan for the possibility that the electorate would vote to leave the EU amounted to gross negligence. It has exacerbated post-referendum uncertainty both within the UK and amongst key international partners, and made the task now facing the new Government substantially more difficult."

What next?

The identities of the claimants and their arguments are set to become clearer by this September, following the court's order after the opening hearing for further documents to be lodged with it. The claimants may pursue separate cases, or they may join together in the same claim.

We understand that a further hearing is scheduled for mid-October 2016. Any appeal could reach the Supreme Court by Christmas.

While the litigation may help resolve the uncertainty surrounding the Brexit process, it is not likely to conclude before 2017. This is going to be one of the most eagerly awaited administrative law decisions for several years and could both clarify the extent of the royal prerogative but also for more pressing matters help shape how Brexit can be taken forward.