I am a self-professed EU nerd. In the past 25 years I have been intimately involved in EU academia, civil service and politics. One of the re-emerging themes of integration is differentiation or core Europe; that is, not all member states need to do the same things at the same time. As it happens, it was the subject of my doctorate at the London School of Economics in 2000.

The debate on core Europe has a tendency to pop up regularly — usually when one or more of five issues are on the Union agenda: the common currency, foreign policy, immigration, enlargement or trouble makers. Time to deepen integration, without the participation of everyone, the argument goes.

There are two basic options. The first is to create a core outside the current institutional framework of the EU. This is a bad idea, and would probably lead to a break-up of the Union. The second is my preferred one: to use the clauses of enhanced co-operation inside the current treaties. They are based on clear rules and are open to all willing and able to advance.

As we approach the 60th anniversary celebrations of the Treaty of Rome in March, all these things are on the agenda so we are bound to see a renaissance of the flexibility debate. The EU summit in Malta this month gave a prelude. In a joint paper, the Benelux countries noted that: “Different paths of integration and enhanced co-operation could provide for effective responses to challenges that affect member states in different ways.” Echoing that declaration, Germany’s Chancellor Angela Merkel said: “The history of recent years has showed that there will be a multi-speed EU, and not all members will participate in the same steps of integration.”

This is not radically new. The process of European integration has always had elements of differentiation. There are hundreds of examples of transition periods and permanent opt-outs scattered around primary and secondary legislation. The euro and the Schengen agreement are prime examples.

As early as 1974, Willy Brandt, then German chancellor, argued that the communities needed graduated integration, otherwise the Union’s cohesion would be threatened. In a 1975 report, the Belgian prime minister, Leo Tindemans, argued that it was not absolutely necessary for all member states to advance at the same time.

The most important contribution on flexibility came in 1994 from Wolfgang Schäuble and Karl Lamers, both then foreign policy experts in the Christian Democrats. They called for the creation of core Europe, based on the founding states (minus Italy) forging ahead with a common currency.

This led to a vibrant debate on differentiation, and ultimately the institutionalisation of enhanced co-operation in both the Amsterdam and Nice treaties. The basic idea was to allow for differentiation inside the EU as long as a few key conditions were met: that the co-operation is open to all and does not damage the essential functions of the internal market.

There are three main reasons for the issue to re-emerge now. The first is the size of the EU; it is impossible for almost 30 member states to advance in unison. Some will always oppose deeper integration — but the threat of being left in a minority is an effective spur for compromise.

The second is the lack of a common vision. With the UK leaving the EU some member states see the core as a possible way to advance. The paradox is that France, which most often raises the idea of core Europe, has historically been the biggest brake on integration. Be it with enlargement, the internal market, trade or moving to majority decision-making, Paris has often been the first to say Non! If Marine le Pen were to be elected president, things could turn from bad to worse.

The third reason is linked to key policies such as the euro, immigration and defence. EU leaders are seeking solutions to the governance of the common currency, the burden-sharing of asylum policy and the potential weakening of Nato with the new US administration. Differentiation is one option, but the practical paths are limited. One lacklustre attempt to establish a financial transaction tax among a selected few members is now as dead as its idea.

Expansion leads to diversity and the greater the diversity, the more the focus on flexibility. In 2002, I ended my book on flexibility by noting that if EU member states are not in agreement on the objectives of European integration, then the debate on a hard core will emerge again. And here we are.

The truth is that core Europe is like a nuclear weapon: you can threaten with it, but not use it. All the elements and rules of flexibility are already in the treaties. If, and this is the big question, the EU member states truly want to deepen European integration, the answer is not a core outside, but enhanced co-operation inside the EU.