The UK Parliament must give its approval before the Government gives formal notice of the UK's intention to leave the European Union, the High Court in London ruled today.

Lord Chief Justice Lord Thomas of Cwmgiedd, Master of the Rolls Sir Terence Etherton and Lord Justice Sales ruled against the Government's claim that it was entitled under prerogative powers to trigger article 50 of the Lisbon Treaty without reference to Parliament.

The case was brought by campaigners who argued that the proposed course was unconstitutional. They claim the move would take away rights conferred under the EUropean Communities Act and subsequent legislation.

Prime Minister Theresa May has said she wants to give the article 50 notice by the end of March 2017. The other EU member states have said that formal negotiations over the terms of the UK's exit cannot begin until the notice has been given.

Ministers' central contention was that when Parliament passed the 1972 Act, it had to be taken to have intended that the Crown would retain its prerogative power to withdraw from the EU Treaties, and thereby intended that the Crown should have the power to choose whether EU law should continue to have effect in UK domestic law.

However, in a single judgment the three judges said there was nothing in the language of the 1972 Act to support this argument, which was contrary both to the language used in the Act and to the fundamental constitutional principles of the sovereignty of Parliament and the absence of any entitlement on the part of the Crown to change domestic law by the exercise of prerogative powers.

The court agreed to certify the case as suitable for appeal direct to the UK Supreme Court, and the Government has since confirmed its intention to appeal.

Commenting on the ruling, Charles Livingstone, partner in Brodies' public law team, said: “Today’s decision is not the end of the matter. 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.”

Click here to access the judgment and a summary.