The EU has drawn a strict red line in the Brexit talks under its demand that the European Court of Justice have jurisdiction over cases in the U.K. that originate before it formally leaves the bloc.

The negotiating position would, if it formed part of the final deal, mean in effect that companies and individuals in the U.K. will be subject to the jurisdiction of the EU’s highest court for years after Britain leaves the bloc — cases can take anywhere from three to 15 years to resolve. Prime Minister Theresa May has been adamant that a key part of leaving the EU is escaping the control of the Luxembourg court.

The decision to draw a strict red negotiating line under the issue came during a meeting of Brexit attachés in Brussels Monday, according to three officials present at the meeting and another one briefed on the discussions.

The group, known in Brussels jargon as the Ad hoc Working Party on Article 50, includes diplomats from 27 EU member countries and meets regularly to thrash out the details of the members countries’ position on Brexit. These then feed through to the European Commission’s negotiating team led by Michel Barnier.

The group agreed that no leeway should be given to Britain on the ECJ’s jurisdiction for cases that are already underway — including cases that have yet to make their way through the British courts and so haven’t reached the European level.

The European Commission is due Wednesday to send a final version of its position on a host of issues to the British government ahead of the first full week of Brexit talks. The update will include a paper on the power of the ECJ and its competence to adjudicate in the period following the British withdrawal date in March 2019.

Changes to the original document will be minor and related to “issues on governance and judicial cooperation,” an official said. 

The original position paper on the ECJ’s jurisdiction post-Brexit, released June 28, read: “The United Kingdom’s withdrawal as such does not deprive the Court of Justice of its competence to adjudicate in proceedings which are pending on the withdrawal date.”

“The line has been drawn and it’s not going to move,” said an EU diplomat present at Monday’s meeting, referring to the ECJ’s jurisdiction. “If anything we reinforced our position.”

Addressing activists at the Tory party conference in Birmingham last October, May was adamant about escaping the court’s influence over U.K. affairs. “Let’s state one thing loud and clear: we are not leaving the European Union only to give up control of immigration all over again. And we are not leaving only to return to the jurisdiction of the European Court of Justice. That’s not going to happen,” she said.

A U.K. government spokesman said Tuesday: “The prime minister has been clear that, after withdrawal, we will take back control of our laws and bring an end to the jurisdiction of the European Court of Justice in the United Kingdom.”

However, in recent days there have been signs the U.K. government is softening its position. May’s official spokesperson confirmed on Monday that the U.K. could be subject to judgments by the ECJ during a Brexit transition “for a limited time.”

“[Brexit] transition rules could involve the ECJ for a limited time, but obviously that is all a matter for negotiation,” a spokesperson for the prime minister told journalists.

Resistance in London to the EU’s position on continuing court involvement could jeopardize the outcome of several ongoing cases, including one brought by the EU’s anti-fraud office (known by its French acronym OLAF) that looks set to be adjudicated by the ECJ. It found earlier this year that Britain turned a blind eye to a massive fraud network that allowed ultra-cheap Chinese goods to flood into Europe.

The U.K. faces a potential €2 billion bill from Brussels after OLAF concluded that British customs played a central role by repeatedly ignoring warnings to curb Chinese textiles and footwear pouring into the EU at a tiny fraction of their cost of production.

Charlie Cooper contributed to this article.