In its Opinion 1/91 of 14 December 1991 on the Treaty establishing the European Economic Area the European Court of Justice (“Court of Justice”) held that the EEC-Treaty, albeit concluded in the form of an international agreement, none the less constitutes the constitutional charter of a Community based on the rule of law.

Accordingly, Article 50 of the Treaty on European Union (TEU) which provides that Member States can notify to the European Council their intention to withdraw from the European Union is a constitutional rule of the European Union and the compliance with Article 50 by the leaving Member State, the European Council and the remaining Member States is subject to legal and constitutional review by the Court of Justice.

A number of constitutional issues which could be subject to review by the Court of Justice in the context of Brexit have been controversially discussed in recent weeks.

One of the most prominent issues is the UK constitutional controversy whether the UK’s Prime Minister could notify pursuant to Article 50 TEU the European Council of the intention of the UK to withdraw from the European Union without obtaining prior approval by the UK Parliament. This issue is currently pending in the English High Court, with the potential for the point of law to go all the way to the Supreme Court.

Since Article 50 (1) TEU provides that a Member State may decide to withdraw from the Union “in accordance with its own constitutional requirements”, the issue of whether approval by Parliament for Brexit is required under the UK’s constitution, turns into an EU constitutional issue as well, since it is debatable whether compliance with the relevant Member State’s own constitutional requirements is a pre-requisite for validly triggering the Article 50 TEU procedure and whether such compliance is therefore ultimately subject to scrutiny by the Court of Justice.

One potential avenue to proceedings in front of the Court of Justice could, depending on the circumstances, be Article 263 of the Treaty on the Functioning of the European Union (TFEU) in case of an infringement of the EU Treaties by the Council engaging in withdrawal negotiations where no constitutionally valid withdrawal notice within the meaning of Article 50 TEU has been given. Another avenue to the Court of Justice would be open pursuant to Article 267 TFEU in case that the question of whether the UK has actually ceased to be a Member State of the EU or not (which it only does if a valid notice pursuant to Article 50 had been given) becomes relevant in any domestic court proceedings (for example within commercial disputes) within any of the other 27 Member States of the European Union under any relevant circumstances.

In this respect one might assume that the Court of Justice would with some likelihood follow any ultimate binding decision rendered by the Supreme Court of the United Kingdom. Accordingly the degree of legal uncertainty which has arisen would be reduced considerably if there would either be an approval by the UK Parliament of any Article 50 TEU notice to be given or an ultimate decision by the Supreme Court of the United Kingdom on that point.

Another “constitutional” issue which is currently controversially discussed is what happens if the UK Government or UK Parliament changes its mind about a Brexit during the two year withdrawal negotiation period.

For a long time it had been the widely held view across Europe that any termination notice by the UK Government to the European Council cannot be revoked unilaterally since the wording of Article 50 TEU is said to be clear – it does not provide for a revocation right and provides that the termination notice kick-starts the two years negotiation period which automatically results in the relevant Member State leaving the EU after such two years unless an extension of the two year period is agreed unanimously by all Member States, and since Article 50 (5) TEU provides that any rejoining of the EU after withdrawal needs to take place pursuant to Article 49 TEU. This view has, for example, been set out in a research paper of the European Parliament published in February 2016.

In contrast to that view , on 4th May 2016 the European Union Committee of the House of Lords issued a report in which it proposes a contrary legal view that a withdrawal notice can be revoked unilaterally during the two years negotiation period. This report is based on a public evidence session which the committee held with Sir David Edward, QC, and Professor Derrick Wyatt, QC.

It should be noted that neither report discusses whether and to what extent Article 68 of the Vienna Convention on the Law of Treaties – which provides that certain notifications which target at ending International Treaties may be revoked at any time before they take effect – could become relevant for the interpretation of Article 50 TEU. In addition neither report considers how Article 50 TEU should be interpreted, in the light of the autonomous approach which the Court of Justice takes in respect of interpreting and applying European law independent from the individual domestic approaches of EU Member States and independent from international law and which might disregard Article 68 of the Vienna Convention. This is because the Court of Justice would argue that the European Union is a constitutional Union which exists outside of and beyond mere international treaties (see for example Opinion 1/91 of the Court of Justice of 14 December 1991).

Again, one potential course for proceedings in front of the Court of Justice could, depending on the circumstances, be Article 263 of the Treaty on the Functioning of the European Union (TFEU). This could arise in the context of an infringement of the EU Treaties by the Council, the Commission or the European Parliament in adopting acts implementing such withdrawal of the termination notice, or, in any event, the Preliminary Ruling proceedings pursuant to Article 267 TFEU as set out above.