Enlargement of the European Union sounds good, but as is often the case in European politics, it’s best to be careful what you wish for. The first enlargement in 1973 was in many ways the worst: the UK, Ireland and Denmark have proved to be difficult members of the club ever since, plagued by second thoughts and encumbered with opt-outs. And Norway, which was lined up to join the EU with the trio, took fright at the last moment and has spent 40 irritating years on the touchline. Today, Britain risks sabotaging again the future of Europe.

Successive British prime ministers have caused trouble by ‘banging on about Europe’, as David Cameron has put it. Harold Wilson engineered a fake ‘renegotiation’ of the UK’s terms of membership in 1975. Margaret Thatcher, insistent on getting her money back in arduous rows over the EU budget, destroyed the concept of autonomous ‘own resources’ – she also steered Britain well clear of the Schengen free travel area. John Major demanded, and got, opt-outs from the euro and EU social policy. Tony Blair then opted out of new EU policies on justice and home affairs, sought to evade the Charter of Fundamental Rights and impeded the development of the EU’s common foreign and security policy.

Since the Conservative Party won a majority last May, David Cameron has flaunted his new renegotiation of the UK’s terms of membership. While leader of the previous coalition government with the ostensibly pro-European Liberal Democrats, and without a thought for his EU partners, Cameron has already installed in Britain’s fragile constitution the EU Act, which imposes a referendum on the hapless British voter every time the EU changes its treaties. And for none other than ideological reasons, he then ruled Britain out of joining the new EU banking union. The prime minister cloaks his latest provocation in the conceit that his reforms will be good for the EU as a whole. The most totemic of his demands is that the UK wins a ‘formal, legally-binding and irreversible’ exemption from the EU’s historic mission of ‘ever closer union of the peoples of Europe’. Jobs and immigration may well stir the masses, but ‘ever closer union’ is the issue that most sharply divides the sovereignists from the federalists, and could, if mishandled, do severe collateral damage to the rest of the EU.

Unfortunately for Cameron, this time-honoured phrase is entrenched in the UK’s original EU accession treaty, and it appears four times in the current version of the EU’s statutes: in the preambles to the Treaty on European Union (TEU) and the Treaty on the Functioning of the European Union, the preamble of the Charter and, most significantly, in Article 1 of the TEU as the Union’s mission statement. Article 1 predicates everything that follows: the EU’s values (Article 2), the EU’s political objectives (Article 3), the general principles of EU law (Article 4), and so on.

Article 1 tells us that European integration is a continuing process. The phrase does not, however, define the ultimate goal of the emerging polity, though over the years it has allowed many intermediate federal steps to be taken. It is not used as a specified legal basis for secondary legislation, and it is not, as is claimed by its British enemies, a ‘ratchet’ clause. But it does serve to establish the constitutional order of the Union. It is a useful and durable means for the European Court of Justice to underline why solidarity and loyalty among member states matter.

There have been proposals in previous treaty negotiations to jettison ‘ever closer union’ in favour of ‘federal vocation’ – proposals that have been blocked not least by the British. To appease David Cameron, incensed at the election in June 2014 of Jean-Claude Juncker to the presidency of the European Commission, the European Council agreed to clarify that ‘the concept of ever closer union allows for different paths of integration for different countries, allowing those that want to deepen integration to move ahead, while respecting the wish of those who do not want to deepen any further.’ Because this was a highly ambiguous statement, it was also unwise. The European Council did not say whether the ‘different paths’ were meant to lead in the same direction or to different destinations. While a multi-speed approach to integration makes obvious sense, allowing some states to integrate and others to unpick integration is nonsense. Can any union survive if its members choose divergent goals?

Clarity about the long term goal could be helpful, even democratic, but care must be taken to avoid tampering with a cardinal provision of the EU treaties in a vain response to the demands of British domestic party politics. Cameron’s colleagues in the European Council are surely aware that making concessions on ‘ever closer union’ could well set a trend, and add to Europe’s risk of disintegration at this delicate moment. A declaration by the European Council to the effect that the Treaty does not, in fact, mean what it says by ‘ever closer union’ would be attacked in the European Parliament and would be certain to be condemned by the Court of Justice.

Nevertheless, a dissatisfied David Cameron is now demanding his legally-binding exemption from ‘ever closer union’. This leaves Brexit negotiators to choose between, on the one hand, conceding to the UK a unilateral derogation from one of the cardinal provisions of the Treaties and, on the other, drafting something more substantial and multilateral that will re-interpret ‘ever closer union’ for everyone else as well. Let us take what Cameron says at face value. He has consistently made the point that what he is asking for may entail treaty change. From that we may infer that he knows that a mere declaration against ‘ever closer union’ will be insufficiently binding. So we need something that is more than a declaration and less than a revision of Article 1. Because time is pressing, here is a suggestion for a new Protocol to be attached to the Treaty of Lisbon whenever it comes up for its next review – on the precondition that the UK referendum has first said ‘Yes’ to Europe.

Nothing in these Treaties commits any Member State to any measure of integration towards the ever closer union of the peoples of Europe to which that State has not consented in accordance with its own constitutional requirements.

Such a passage has the merits of respecting the corpus of EU law, of treating all member states as equals, of not trashing the Union’s mission, and of not opening up an intractable controversy about the nature of Europe’s eventual polity. It would prevent a weakening of the ties that bind, and avoid enfeebling the European Union at a time when it needs to be stronger than ever – with or without the British.