The UK High Court ruled yesterday that the Government cannot rely on its historic prerogative powers to launch the Article 50 process to withdraw the UK from the EU. The Government has said it will appeal the decision to the Supreme Court but, if unsuccessful, the Government will effectively be required to seek Parliament’s approval before starting the two-year negotiating process to withdraw from the EU.

The Government argued that it could rely on historic ‘prerogative powers’ traditionally used to conduct international relations and to make (and unmake) international treaties. In court, the Government accepted that once the Article 50 notification had been made it could not be revoked. The Government recognised that this could lead to the UK’s ejection from the EU if no deal is reached within the two-year period set down in the Treaty. In turn, that would mean the European Communities Act 1972 – enacted by Parliament to give effect to the UK’s membership of the EU – would no longer apply.

The Government said that Parliament must be taken to have intended the exercise of the royal prerogative in this way when it enacted the 1972 Act. The UK high court, including the UK’s most senior judge, disagreed. They thought the Government’s arguments were “contrary [to] both the language used by Parliament in the 1972 Act and to the fundamental constitutional principles of the sovereignty of Parliament and the absences of any entitlement on the part of the Crown to change domestic law by the exercise of its prerogative powers.”

The Government has already stated that it will appeal the judgement which will be heard by the Supreme Court over a 4 day session in the week beginning the 5th December 2016. It is worth noting that this case has proceeded, and is likely to proceed in the Supreme Court, on the basis, accepted by both parties, that an article 50 notification cannot be revoked, once made by the UK. However this is itself an open legal question, and some prominent voices, including Donald Tusk, President of the European Council, have recently indicated that this may not be the case. If unsuccessful the Government will have to decide on how it seeks Parliamentary approval to start withdrawal negotiations – a step that will provide Parliament an opportunity to demand a greater say in shaping those negotiations and may delay their start.

The full judgment can be found on the judiciary website with accompanying transcripts.