Conservative lawmaker Jacob Rees-Mogg said that if Britain is “stuck” in the European Union, “we must use the remaining powers we have to be difficult.” (Simon Dawson/Bloomberg News)

Despite its best efforts, Britain is still in the European Union. And because of an agreement with European leaders this week, the country is likely to stay in the bloc until Oct. 31, more than three years after it voted to leave.

To Brexit hard-liners, that delay is a scandal. And some European leaders are concerned that the longer extension could give anti-E.U. voices in Britain an opportunity to sabotage and disrupt the supranational organization.

Notably, Britain is now expected to participate in elections to the European Parliament in late May. Nigel Farage, former head of the pro-Brexit U.K. Independence Party, has pledged to fight in these Europewide elections. “It’s time we taught them a lesson,” he said, referring to E.U. leaders.

Other Brexiteers have hinted at a broader strategy of disruption. Conservative lawmaker Jacob Rees-Mogg said that if Britain is “stuck” in the E.U., “we must use the remaining powers we have to be difficult.”

For bloc members that hoped that the E.U. could form closer bonds in the near future, that is a disquieting possibility. “The key for us is to remain grouped together,” French President Emmanuel Macron said. “We have a European renaissance to lead. We don’t want the Brexit problem to block us on this point.”

Behind closed doors on Wednesday evening, Macron spoke at length with other E.U. leaders of his concerns about a continued British presence, according to diplomats familiar with the conversations.

But despite the best wishes of Rees-Mogg and others to make life difficult for the E.U., doing so may be harder than they anticipated in practice.

One senior European diplomat, talking ahead of Wednesday’s summit, said that a recent E.U. analysis concluded that Britain would not obstruct the bloc. “They haven’t done that over the past two years,” the diplomat said, speaking on the condition of anonymity to discuss sensitive internal government thinking.

May has pledged that Britain will remain a functioning member of the E.U. while it is still in it, with the Guardian reporting Thursday that she reminded E.U. leaders that Britain is a “serious country” and that the most intransigent critics of the bloc in Britain are not currently part of the government.

Even if Britain wished to disrupt, the E.U. calendar is not in its favor: There is simply not much major legislation that would require unanimity that Britain could block. “If it would be a normal legislative period, it could have been different,” the diplomat said, “but it is not.”

“It’s not serious,” the diplomat said of Rees-Mogg’s tweet.

Although Brexiteers have long formed a sizable part of the contingent of lawmakers sent to the European Parliament from Britain — in part because persistent low turnout allowed anti-E.U. voters more sway — the circumstances of Brexit may change that.

A poll conducted for Politico Europe in January found that traditional parties such as Labour and the Conservatives would still dominate the vote, with UKIP likely to lose seats. One group of independent politicians in Britain’s Parliament who oppose Brexit said last month that it had registered a party that could compete in the European elections.

Even so, E.U. critics may still try to cause problems, even if their hands are tied. On Twitter, Rees-Mogg responded Wednesday to criticism from Michael Roth, Germany’s minister for European affairs, who had called on him to take a “constructive approach” to Brexit.

“A Eurocrat thinks it is out of order to stand up for democracy,” Rees-Mogg wrote, “it is typical of their high handed approach and encourages us to be difficult.”

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