Last week’s decision of the Court of Justice of the European Union in Home Secretary v CS marks a new high water mark in the Court’s expansion of the implications of EU citizenship and the role of the EU Charter of Fundamental Rights. It may also raise important political questions for Government ministers.

CS, in whose favour the Luxembourg court has just ruled, is fighting deportation from the UK. She is the daughter-in-law of Abu Hamza, and has been convicted of attempting to smuggle to him whilst in prison. That gives this case a notoriety. But even without this headline factor the new legal principles revealed would justify wide attention.

CS is a Moroccan national. In 2002 she married a British man, and was then granted a visa to enter the UK on the basis of that marriage. In 2012 she was sentenced to 12 months imprisonment for attempting to smuggle a SIM card into Belmarsh Prison for her father-in-law. Under English law it is mandatory under the UK Borders Act 2007 for the Home Secretary to make a deportation order in respect of any person sentenced to at least 12 months imprisonment. CS challenged the deportation order by a range of arguments, but the only one relevant for present purposes is that as to EU law. The Upper Tribunal (Immigration & Asylum Chamber) sought a preliminary ruling from the EU Court.

Since CS is a Moroccan citizen, not a citizen of any EU state, and since she has not sought to move between EU countries, it may at first sight seem hard to understand the potential application of EU law. Her argument depended on the fact that the year before her conviction she had given birth to a child, and that she was now separated from her husband. So in practice, she argued, if she has to leave the UK, her son will have to leave with her.

The young son, having been born in the UK, is a UK citizen. In consequence he also enjoys the status of EU citizenship. The meaning of EU citizenship is defined by article 20 of the Treaty on the Functioning of the EU as entailing four rights: (i) free movement within the EU; (ii) voting in European Parliament and municipal elections; (iii) consular protection when outside the EU; (iv) the right to petition EU institutions. None of these rights of the son is threatened in any obvious way. If he leaves the UK with his mother now, he will be able to return at any time.

Yet the Grand Chamber ruled that article 20 precludes the UK’s legislation. Only a genuine, present and serious threat to the fundamental interests of UK society could, in the Court’s ruling, render the deportation of the primary carer of an EU citizen child compatible with EU law. How did the Court reach that conclusion?

The explanation lies in the growth of a distinct Luxembourg jurisprudence on EU citizenship. Its genesis can be traced to a remark in the Grzelczyk case in 2002: it was there held to infringe the treaties if a subsistence benefit, which would have been granted to a Belgian student, was not also granted to a French student. The judgment in what may seem a relatively routine equal treatment case was decorated with this pronouncement:

“Union citizenship is destined to be the fundamental status of nationals of the member states, enabling those who find themselves in the same situation to enjoy the same treatment in law irrespective of their nationality, subject to such exceptions as are expressly provided for.”

The opening clause of that proposition has been repeated innumerable times by the Court, and recently ripped from the remainder of the sentence so as to apply it where the full original principle could have had no possible application. The British Advocate General Sir Francis Jacobs memorably remarked that an EU citizen could go anywhere in the Union and say civis europeus sum; his listeners probably did not realise that he was thereby evoking a famous speech of that most imperialist of statesmen, Lord Palmerston.

A landmark in this development was the case of Rottmann, which provoked expressions of severe unease from senior British judges. On its facts this was an unusual case. Rottmann was an Austrian national who applied for, and was granted, naturalisation as a German national. Under Austrian law he thereby lost his Austrian nationality. The Germans then discovered that he had obtained his German citizenship by deception, having concealed the fact that he was under investigation for fraud in Austria. The Germans revoked his German citizenship. So he was left with no citizenship at all. The CJEU, reciting the “fundamental status” dictum, held that whilst it was open to the German authorities to reach a decision to withdraw nationality, the concept of EU citizenship required there first to be a proportionality balancing exercise of many factors.

Laws LJ found the Rottmann decision so difficult that he doubted whether the European Communities Act 1972 would have conferred on a British court any authority to follow it:

“The conditions on which national citizenship is conferred, withheld or revoked are integral to the identity of the nation state. They touch the constitution; for they identify the constitution’s participants.”

In two subsequent cases a number of justices of the UK Supreme Court have, without expressing a concluded opinion, leaned the same way. In HS2 Lords Neuberger and Mance said it was certainly arguable that there might be fundamental principles of domestic law of which the 1972 Act did not authorise abrogation. In 2015 Lord Mance developed his thinking in a long passage in the deportation case of Pham: he said that unless UK law had conferred on the CJEU “unlimited as well as unappealable power to … expand the scope of European law” (which he plainly doubted) a domestic court must ultimately decide the limits of the competence of the European institutions. This issue of the boundary of the effect of 1972 Act remains unresolved.

The emotional resonance of statelessness to a central European with a sense of history may not always have occurred to British critics of the Rottmann decision. A right to nationality was one of the rights proposed by Hersch Lauterpacht in his wonderful book An International Bill of the Rights of Man. So special facts could explain and justify Rottmann as an outlier. But no such special features can be invoked to explain the developments which followed.

In Ruiz Zambrano a Columbian national and his wife entered Belgium. They were refused asylum but remained in the country. The husband obtained work, although he had no work permit. Whilst in Belgium two children were born to the family. The husband lost his job. Entitlement to unemployment benefit depended on a certain period of lawful employment, and by reason of the absence of a work permit he was not qualified. Lacking any other basis for complaint his lawyers asserted that this situation infringed EU citizenship rights.

In a long opinion notable both for its eloquent rhetoric and remarkable conclusion, Advocate General Eleanor Sharpston agreed. Although the husband himself had no EU citizenship rights, the two children did have.  Although there was no cross-border element – none of the family had moved, or wanted to move, elsewhere in the EU – she argued that EU citizenship could affect a purely internal situation. She said that unless the husband was granted a work permit, the family would have no practical alternative to leaving, and this would mean that the children would be deprived of genuine enjoyment of the substance of their EU citizenship rights. The Court adopted this conclusion.

This, then, was the state of the EU citizenship jurisprudence when CS was held. The Grand Chamber effectively held that the Zambrano principle led to the conclusion that CS’s deportation would infringe EU law because its practical result would be to deprive her son of the enjoyment of his EU citizenship. In two respects, however, the CS judgment went further than Zambrano. In Zambrano the national government had a discretion: it was always open to the Belgian authorities to grant the husband a residence permit and work permit. In CS, by contrast, the deportation was mandatory under an Act of Parliament.

Secondly, in CS the Court reached its conclusion only by invoking the right to private and family life in the EU Charter. In this respect the Court differed from the reasoning of its Advocate General, and also from the thinking of AG Sharpston, who had expressly – albeit reluctantly – rejected the possibility of the EU Charter expanding the meaning of the citizenship rights for the benefit of Zambrano’s family, since the Charter could not create new free-standing rights. By its own language the EU Charter is applicable only where member states are “implementing EU law”.   Since the UK cannot sensibly be said to have been implementing any fragment of EU law when it enacted the law on deportation of third country nationals, bringing the Charter into play involves a high degree of intellectual invention.

The ratio of CS seems to be that it can be contrary to article 20 for a member state to take any action at all in respect of A, being a non-EU citizen, which has the practical effect of depriving B, who is an EU citizen, of the practical enjoyment of citizenship, even for a temporary period. It seems odd to regard the young son, who remains a UK/EU citizen, to be deprived of anything of value on the scale of fundamental rights: in today’s world children go to live abroad with their parents in a host of circumstances. The implications, moreover, are so potentially wide-ranging that it is hard to see any outer boundary of their reach. Consider, for instance, the sentence of 12-month imprisonment on CS. Suppose defence counsel in the Crown Court had produced evidence that the only practical care arrangement for the baby son would be with an aunt in Morocco. Would that have meant that any imprisonment of CS would infringe EU law?

The Luxembourg Court having answered the questions posed for preliminary ruling, CS’s case now returns to the UK’s Upper Tribunal. There may be considered to be a significant political decision to be taken by the Home Secretary as to the line now to be submitted. In Pham, the UK Supreme Court strongly suggested that UK courts would not give effect to an EU Court decision on the existence or effect of UK citizenship. If the expansion of the European jurisprudence of EU citizenship is in that respect beyond the domestic effect allowed by the European Communities Act, may it not be arguable that the Zambrano/CS expansion of the effect of EU citizenship is also beyond it? Although there is a statutory exception to mandatory deportation where it would breach the foreign criminal’s rights under EU treaties, that cannot assist CS: for CS has no such rights, it is only the son who does. So mandatory deportation can be avoided only if sections 2, 3 of the European Communities Act 1972 trump it. So should not the Home Secretary be considering arguing that in our domestic tribunals the CS judgment should be ignored?

Even if the Government shrinks from so head-on a conflict, it may surely adopt the policy that in any Brexit negotiations of movement or EEA rights for EU citizens every trace of the impact of both EU citizenship and the EU Charter should be excluded.