EU Law Limits to State Discretion
At first the Brexit case of loss of Union citizenship does not seem to fall within the remit of European law precisely because it would occur simultaneously to the UK’s withdrawal, but appearances deceive. The jurisprudence of the European Court of Justice has imposed certain limitations on the autonomous powers of the member States to determine issues relating to the acquisition and loss of nationality. Generally speaking, member states need to pay ‘due regard to Community law.’ This has been the obiter dictum since the European Court of Justice started to express itself on the matter in the early Nineties, with the case of the Argentinian dentist Micheletti.12
On that occasion, the European Court of Justice concluded that:
under international law, it is for each Member State, having due regard to Community law, to lay down the conditions for the acquisition and loss of nationality. However, it is not permissible for the legislation of a Member State to restrict the effects of the grant of the nationality of another Member State by imposing an additional condition for recognition of that nationality with a view to the exercise of the fundamental freedoms provided for in the Treaty.13
Besides public international law, member states are to respect general principles of European law that applies to matters of loss of Union citizenship, including the principle of proportionality - that has attracted much interest in the wake of Rottman - but also the principle of equal treatment, the principle of protection of legitimate expectations and the principle of sincere cooperation (Gemeinschaftstraue).
In Rottman, the European Court of Justice acknowledged that member states may legitimise the revocation of nationality and its consequences for the status of European citizens for the protection of their special relationship with their nationals, based on ‘solidarity and good faith, and the reciprocity of rights and duties, which forms the bedrock of the bond of nationality.’14 Such legitimate interest is, however, subject to the test of proportionality and other limits. The European Court of Justice left it up to the national court to decide whether it would be proportional to withdraw German nationality. Even after Rottman it is compatible with European law to deprive naturalised of citizenship in case of fraud - which is also consistent with international law - but member states:
must not solely adhere to this principle of proper administration in accordance with their own internal (administrative law), where exclusively national points of view will be dealt with (...); but then a second proportionality test must be applied regarding this loss of the fundamental status of Union citizen. That element can, of course, already be included in the proportionality test by national law, but to the extent that it is not the case, it must be examined separately. (Jessurun d’Oliveira 2011, p. 8)
For completing the proportionality test the European Court of Justice provides a number of criteria to take into consideration: The position of family members who might lose rights of residence; the seriousness of the fraud involved; time elapsed between naturalisation and deprivation; if the person may recover prior nationality. It has been stressed that the principle of proportionality ought to be paramount to all decisions on loss, including those where loss occurs automatically (ex lege). Since Rottman, it is clear that member states are subjected to the general principles of European law in matters such as loss of Union citizenship.
Nationality law belongs to the domaine reserve but domestic choices are not neutral vis-d-vis Union citizenship. There may be incentives for the UK to adopt or modify domestic provisions in a way, however, that would need to pay ‘due regard’ to European law.15