The ‘roadmap’ released by Vote Leave last week claims that triggering Article 50, the formal mechanism for leaving the EU, would not be the only legal option in the event of a Brexit vote, citing alternatives of the ‘Greenland example’ or use of international law.

An analysis of the roadmap by Kenneth Armstrong, Cambridge Professor of European Law, questions the credibility of these claims, which he describes as either “legally implausible” or “politically less attractive” than Article 50.

He also argues that Vote Leave’s proposed legislation to diminish EU laws within the UK during withdrawal negotiations may actually weaken the sovereignty of the Westminster Parliament, as it would provoke direct constitutional challenges from the UK’s devolved governments as well as UK courts.

The analysis is published as a working paper on the University’s Centre for European Legal Studies website.

Under Article 50, as set out in the Lisbon Treaty, the UK would remain an EU member state for two years while negotiations take place. There is a possibility for the timescale for negotiations to be extended, says Armstrong, but any EU state could veto any extension forcing the UK into a take-it or leave-it dilemma as the clock runs down.

Vote Leave argue that Brexit could utilise alternative legal procedures to extricate the UK from the EU, citing the example of Greenland’s EU withdrawal, or using the Vienna Convention of international treaty law.

However, both of these supposed options predate Article 50, which would now override them, says Armstrong. In addition, the Greenland example doesn’t hold up, as it was not a Member State in its own right, but a ‘constituent territory’ of Denmark. It would also mean invoking Article 48, which would require the unanimous agreement of every member state (unlike Article 50), and so create veto opportunities.

“It is simply not obvious why Vote Leave would consider this to be a viable or useful alternative to the Article 50 process itself,” said Armstrong.

The 1969 Vienna Convention of international law sets out a framework for negotiation and termination of treaties across the globe, and Vote Leave argue that this earlier international agreement also makes it possible to leave the EU.

However, the Vienna Convention only applies to treaties of other international organisations, such as the EU, “without prejudice to any relevant rules of the organisation”, such as Article 50.

“There is simply no way that the European Court of Justice would permit the autonomous legal order of the EU and its established mechanism of Article 50 to bend to international law in this manner,” said Armstrong.

“The clear conclusion is that Article 50 is the correct legal basis for the conduct of a withdrawal process. Neither the experience of Greenland nor the Vienna Convention casts any doubt on that conclusion,” he said.

Vote Leave’s roadmap also includes a proposed Bill to deviate from EU laws during withdrawal negotiations, to be enacted by 2020. The proposed Bill includes the power to remove EU citizens deemed to be “not conducive to the public good”, and a reigning in of the EU Charter of Fundamental Rights: the section of EU law that concerns human rights.

Armstrong points out that any proposal to introduce a version of this Bill in the current parliamentary session would not just encounter significant obstacles in both the Commons and the Lords, but, if enacted, would create constitutional conflicts between the devolved governments of the UK and the Westminster Parliament.

“It is at least arguable - and one would expect the Scottish government, for example, to argue - that any attempt to limit the EU Charter of Fundamental Rights would require the consent of the devolved parliaments, as otherwise possible legal action against ministers in devolved governments could occur for acting in breach of EU law,” said Armstrong.

He also points out that, while the UK remains an EU member state, which it will do for at least two years following Brexit, the UK courts will be bound by EU laws. Any amendment to the European Communities Act, which establishes the supremacy of EU law, would bring UK courts into a constitutional conflict between obligations under EU law with those under domestic law.

“It is simply not obvious how any given judge, or any given court, would seek to resolve that conflict. Either way, it would antagonise and politicise the judiciary in a manner that many would find unacceptable and could, paradoxically, give rise to judicial challenges to parliamentary sovereignty,” said Armstrong.

“Far from restoring the sovereignty of parliament, its sovereignty could be brought into direct constitutional challenge.”

More videos featuring experts from the Cambridge Centre for European Legal Studies on key EU Referendum questions can be viewed here.