French president François Hollande (L) and German Chancellor Angela Merkel | Martin Bureau/AFP/Getty Welcome to a two-speed Europe Desperate to keep the UK from leaving, Europe is inching towards a dual-track reality.

BERLIN — Europe’s leaders emerged from last week’s marathon session with David Cameron hoping they had just avoided a messy divorce.

They may yet discover that staying together can be just as traumatic.

Most of the focus in the wake of the deal has been on whether British voters will endorse the plan hammered out at the Brussels summit. As important as the deal is to the U.K., its impact on the EU may be even greater. Put simply, the agreement effectively rewrites the rules for member states’ dealings with Brussels.

“What it does is set a precedent that everything is up for renegotiation,” one Brussels official said.

Until now, members have generally been compelled to accept the full canon of EU rules and regulations, notwithstanding a few notable exceptions, such as the U.K.’s right to opt out of the euro.

The EU’s agreement with London goes much further, however, creating the political equivalent of an open marriage. While the two sides would retain their legal union, they wouldn’t be chained to one another. Such arrangements may sound good in the abstract. In practice, they can prove tricky.

After the summit, Cameron left no doubt about what the deal would mean: "Britain will be permanently out of ever-closer union, never part of a European superstate … Britain will never join the euro and we've secured vital protections for our economy.”

Welcome to a two-speed Europe.

The idea of a closely integrated core Europe surrounded by more loosely linked affiliates isn’t new. Many would argue the dual-track already exists with the eurozone. But nearly all the non-euro countries, including all EU members in Eastern Europe, are obliged to work towards membership in the currency area. So to the degree that the two-speed Europe exists, it has been, at least nominally, temporary.

The EU’s proposed arrangement with the U.K. would go a step further by institutionalizing the so-called à la carte approach to membership.

Instead of a core group of like-minded countries coming together to embrace closer integration, one country is pulling way, opening the door for others to do the same.

With nationalism on the rise across much of Eastern Europe, it’s not inconceivable that countries such as Hungary and Poland will also ask for exemptions from joining the euro or other aspects of the EU.

Over time, such moves could further undermine public confidence in the EU, eroding its legitimacy.

Two speeds, no agreement

Many die-hard Europeans have long-argued a two-speed solution is the only way forward. Influential European politicians from Germany’s Wolfgang Schäuble to Italy’s Romano Prodi have endorsed the idea. The challenge has been in agreeing on what it means.

There’s consensus, for example, that the eurozone needs a finance minister. When it comes to defining the role, however, Berlin, Paris and Rome remain far apart.

So far, steps toward a two-speed Europe have been made under duress, such as when the eurozone created a bailout fund. With the common currency under the threat of collapse during the debt crisis in 2011, the then-17 eurozone countries agreed to pool resources to create the bailout fund. The U.K. refused to participate, however, forcing the euro countries to conclude the agreement outside the European treaty.

Unless the Stay camp wins by a wide margin, Brexit will likely remain on the political agenda in the U.K.

Now, desperate to keep the U.K. from leaving, Europe is again inching toward the two-speed reality.

But this isn’t the model Europe’s idealists had hoped for. Instead of a core group of like-minded countries coming together to embrace closer integration, one country is pulling way, opening the door for others to do the same.

Even if the British voted to remain in the EU, it’s far from certain the agreement would actually work.

One aspect of the deal, for example, would allow London to object to regulatory moves in the eurozone that it believes could place the City at a disadvantage. But the remedy for such disputes is vague, suggesting they could easily escalate, again pushing the relationship to the brink.

More generally, unless the Stay camp wins by a wide margin, Brexit will likely remain on the political agenda in the U.K., especially given the support the Out camp has received from popular establishment politicians.

Time for integration?

In some respects, a positive outcome would be the perfect moment for the core EU to pursue integration. One factor holding countries like Germany back has been the fear of offending the U.K.’s sensibilities. By clearly defining the U.K.’s position, the deal would appear to remove that obstacle.

The question is whether the U.K. would remain sanguine about a more tightly integrated EU once it became a reality or see it as a threat.

For now, that scenario remains remote. Europe, divided by the refugee crisis and its economic travails, is in no position to pursue “ever-closer union.”

Despite those challenges, many Europhiles welcome the deal. The EU has been hobbled for more than a decade by uncertainty over the U.K.’s role and the pace of integration, they argue. Even if more countries follow its example, there is no other way for Europe to move forward.

“There will still be a core. The rest will opt out of certain aspects and won’t have as much influence, but I don’t think it’s that dramatic,” said Thorsten Benner, co-founder and director of the Global Public Policy Institute, a Berlin-based think tank. “That’s sensible and it’s the only way this can work.”