London (CNN) The UK's failure to leave the European Union has created an almighty headache in Brussels.

Three years since the UK voted to Brexit have led to nowhere useful. Having missed two deadlines already, Brits are now obliged to take part in European elections on May 23 -- two months after they were supposed to be shot of the EU once and for good.

In two weeks, voters across the Union will elect 751 Members of the European Parliament. Historically, turnout in these elections is low. The small numbers that vote often do so to register their protest.

The anger at Brexit not yet being delivered plays into the hands of euroskeptics. Brexit was scheduled to have happened on March 29 and the UK was supposed to have nothing to do with these elections.

In 2014, Nigel Farage's anti-EU UKIP came first place in the UK's European elections. This put euroskeptic pressure on the then Prime Minister David Cameron, and ultimately forced him to call for the Brexit referendum. And we all know what happened next.

In 2019, Farage's new Brexit Party has an even easier job. Brexit betrayal is a powerful message, and Farage is once again expected to finish first.

Euroskeptic parties won't do well only in Britain. The most recent polling from Europe Elects predicts that these parties could win up to 35% of seats in the European Parliament. Previously, euroskeptics have failed to coordinate at a European level, meaning their level of influence within the EU was always limited.

That could be about to change. Matteo Salvini, Italy's Deputy Prime Minister and leader of the far-right Lega Nord, which governs in coalition with the populist 5 Star Movement, has for months been trying to bring all euroskeptic parties together. If successful, he could create one of -- if not the -- largest bloc inside the EU parliament.

Euroskeptics have historically had less in common with one another than they've had in common, so it's unclear if Salvini can pull this off.

Catherine De Vries, Professor of Political Behaviour at the Vrije Universiteit Amsterdam, thinks that the current momentum behind the skeptics could lead people setting their differences aside.

"If they all do well... some might join it (Salvini's block), because there is an incentive to join a bigger group."

That incentive is the ability to influence EU policy, rather than sit on the margins and protest.

Matthew Goodwin, Professor of Politics at the University of Kent and co-author of the book "National Populism," says many European populists have shifted from wanting a Brexit of their own to believing instead that they can influence Europe from the inside.

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"What they will be is a more disruptive political force within the EU parliament," Goodwin said. "Most likely pushing for the return of powers to nation states, stronger border security, more help with dealing with refugees and probably more democratic and transparent institutions."

Goodwin says the battle between the European mainstream and the anti-EU populists is only going to accelerate as there's nothing to suggest that it's a short-term protest.

"The long-term trends that are driving these changes -- increasingly detached political institutions, a high rate of immigration, strong feelings of economic and status loss among white workers, weakening bonds between old mainstream parties and voters -- these are deep-rooted and have been coming for years, if not decades," Goodwin said. "...(Populism) looks to have much stronger anchors in our society than many are willing to think about."

The European elections could well result in some immediate pain for those in Brussels. But it won't end there.

There is an established pattern in European politics of center-right parties borrowing populist ideas to cling onto voters. The most obvious is example is the 2014 UK Independence Party (UKIP) win leading to the Conservatives offering a referendum on European Union membership.

But, as De Vries explains, it's sometimes more subtle. She points out that Mark Rutte, Prime Minister of the Netherlands, has recently been "downplaying the significance of the EU elections."

Why? Because polling suggest that his nationalist rivals, the Forum for Democracy, could well defeat his governing People's Party for Freedom and Democracy.

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This would give a party that has campaigned for a Dutch exit from the EU, tighter border control and a ban on Islamic face veils a credibility it has yet to achieve. So, downplaying the impact of these elections, making it look like nothing more than a protest vote, suits Rutte.

If he can couple this with soft aping of the populist rhetoric, even better. And Rutte has form here, after he moved more to the right in 2017 in response to pressure from rivals.

Slowly, as mainstream, governing parties borrow the positions of the fringes, the dial shifts and we eventually arrive at a new normal. And this is where it all gets a bit complicated for Brussels.

Fifteen years since the last major expansion of the Union -- when 10 new countries were welcomed to the fold -- the vision of ever-closer union has lost some of its shine.

It's possible that the drive for less Europe has already begun.

"The negotiation has started. The question is how far will those concessions go? How much will the mainstream give to the populists?" Goodwin said.

It could be in the EU's interests to address this new reality, before it's too late. "It all depends on how strong they get. Having 30-35% of seats is one thing; having 40-45% in 10 years is another," Goodwin adds.

And this is the key point: Euroskepticism might have sparked Brexit in the UK, but right now in other European nations it means remaining in the EU and reforming it. And if the EU is to stick together, its leaders would be wise to accept that this might be their new reality.

After all, populists are currently in government in places like Italy, Austria, Estonia, Hungary, Poland, some states of Germany and even the US.

As Goodwin says, they are "inching into the corridors of power," giving us case studies in what populists forming policy actually looks like.

After the parliamentary elections, the EU will elect new presidents of the European Parliament, European Commission and European Council.

Where their predecessors will be remembered for trying to keep the union on the same page as it navigated Brexit, the biggest threat the EU has ever faced, these new leaders may find themselves having to tussle with a bunch of politicians more belligerent than the UK. But, unlike the Brits, the EU will be stuck with them for a very long time.