The High Court of England and Wales has today decided that the UK Government does not have the legal authority to trigger Article 50 of the Treaty on European Union (by which a Member State gives notice of its intention to leave the EU) without first getting the approval of Parliament.

The Government argued that the power to make and break treaties, including the treaties that make the UK a member of the EU, is part of the royal prerogative. Prerogative powers belong to the Crown but are exercised by the Government. While Parliament has removed a number of prerogative powers over the centuries, the ability to conduct international relations (including the power to make and break treaties) remains in the hands of the Government. The Government's position is therefore that it has the power to trigger Article 50 without Parliament having to be involved.

While there is debate on the point, the Government's position is that Article 50 cannot be revoked once triggered. The arguments in the case, and the judgment, therefore proceeded on the premise that triggering Article 50 would inevitably lead to the loss of EU rights incorporated into UK law by the European Communities Act 1972, and so to changing domestic law. The Government argued that the 1972 Act did not say anything that would limit the Government's prerogative power. However, the general rule is that the prerogative cannot be used to change domestic law, and the High Court has agreed with the claimants that this rule prevents the Government from exercising Article 50 without Parliament authorising it to do so.

Today's decision is not the end of the matter, however.

An appeal to the UK Supreme Court was expected whatever the High Court decided, and indeed the Government has already announced that it will appeal. It is very likely that the appeal process will be expedited, which could mean the Supreme Court hearing arguments and potentially even giving judgment before Christmas. The Government's proposed timeframe for triggering Article 50 by the end of March 2017 is therefore unlikely to be prejudiced, as long as the appeal succeeds. However, that may have to change if the Supreme Court also finds against the Government. While the claimants did not seek to injunct the Government from proceeding with its Article 50 plans, today's declaration that it would be unlawful for it to do so will, if confirmed by the Supreme Court, surely mean that time will have to be found to get Parliament's approval for the triggering of Article 50.

The exact form of that approval would be of close interest to the Scottish Government, which opposes Brexit. There is a strong constitutional principle, reflected in the Scotland Act 2016, that Westminster will not legislate on matters that are devolved to the Scottish Parliament without its consent. On the premise of today's decision, legislation to authorise the triggering of Article 50 could be interpreted as inevitably leading to a change in the application of EU law in Scotland, including in devolved areas.

There could therefore be further constitutional arguments about whether the Scottish Parliament's consent should be sought before the UK Parliament passes legislation permitting the UK Government to trigger Article 50. That principle and the relevant provision of the Scotland Act 2016 are generally not thought to create a legal obligation that can be enforced in court, but that has never been tested: it is not inconceivable that further challenges might follow in the event that the Supreme Court upholds today's decision.