How will Brexit impact citizenship?

These consequences concern the European citizenship that each national of an EU member state holds. This citizenship, introduced by the Treaty of Maastricht, gives holders the right to reside, study and work in the territory of the EU. It is a fundamental token of a European Union that serves individuals, rather than states.

European citizenship does not substitute for or take away from the citizenship that individuals hold in their respective member state – it gives additional rights. Citizenship is closely associated with human dignity – it is the basis of being included in a political community. Not being excluded from that community against one’s will, not to be made the object of public authority, is the expression of dignity – the highest value.

Brexit need not strip UK nationals of their European citizenship. Both international law and EU law are very clear.

Of course, a member state is free to terminate its membership for the future, but it cannot extinguish the citizenships that have already been created and the rights that have been exercised – these continue. This status cannot not be taken away neither by the European Union nor by one of its member states.

This is also the impetus of the international law of treaties laid down in the 1969 Vienna Convention on the Law of Treaties. This international law will be binding on the EU, the UK and the remaining member states after Brexit. It governs in considerable detail the consequences that the withdrawal of a state from any treaty, including the Founding Treaties, entail.

One consequence is that the treaty ceases to bind, but the other is that the withdrawal must not have retroactive effect on the rights of individuals already created at the time of withdrawal.

Finally, this has been long-standing policy and law of the UK. When the UK recognised Ireland as an independent country, it made plain that individuals should not be stripped of their UK citizenship against their will as a result. It therefore enabled them to retain their UK citizenship if they so wished. All it took was an expression of will, and the rights to vote for the Westminster Parliament as well as the rights of abode and of working inherent were safeguarded.

Parliaments ultimately are responsible for the rights that individuals hold. The parliaments, of the EU and the UK, are thus the principal guarantors of this continuing citizenship. The Withdrawal Agreement that the EU and the UK have to conclude under Article 50 is under their ultimate responsibility. It is also their responsibility to determine the rights of those individuals that had not yet acquired European citizenship at the time of Brexit, for instance through an associate citizenship for British nationals.

It is a matter of political will, rather than legality, whether UK nationals should get to keep their European citizenship after Brexit. It is up to Theresa May and her government to negotiate it into the withdrawal agreement.

• Prof Volker Roeben is Professor of International Law and Global Regulation at the University of Dundee. He wrote a widely acclaimed report on the European Citizenship for UK nationals post-Brexit.

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