Britain's Prime Minister, Theresa May stands with European Commission president, Jean-Claude Juncker at the front door of 10 Downing Street in London | Carl Court/Getty Images Brexit Files Insight Britain’s Brexit denial How did the May-Juncker dinner go? ‘Badly. Really badly.’

EU leaders expressed mounting alarm Friday that U.K. Prime Minister Theresa May and her team are in a dangerous state of denial about the consequences of leaving the bloc.

The worry over Britain’s unrealistic expectations was a main topic of discussion at background briefings all across Brussels' European Quarter on the eve of an extraordinary European Council summit on Brexit.

Saturday’s summit is the first official gathering of the 27 EU leaders without Britain since May sent a letter formally triggering the Article 50 withdrawal process in late March.

Diplomats said the concerns were tied directly to a dinner that May hosted in London Wednesday night with European Commission President Jean-Claude Juncker, the EU’s chief negotiator Michel Barnier, and other senior officials.

Participants in the dinner were extraordinarily tight-lipped about it afterward, but an EU diplomat said Juncker called Chancellor Angela Merkel at 7:30 a.m. the day after the dinner. And in a speech to the German parliament on Thursday, Merkel sent a pointed wake-up call, saying the U.K. “cannot and will not have the same rights” as EU members and that Britain should have “no illusions.”

May, responding to Merkel’s comments, said that the 27 “were lining up to oppose” the U.K., and she noted, “We’ve seen that actually there will be times when these negotiations are going to get tough.”

One senior EU official said there was some relief that even hard-line Brexiteers were no longer suggesting that leaving the EU without a formal withdrawal agreement might be a good idea. Officials on both sides generally agree that would lead to chaos.

But EU officials, who in recent months have worked hard to build uncharacteristic unity among the 27 on Brexit, suggested it was not clear the U.K. had come to grips with the fact that EU businesses, particularly in the financial sector, were likely to suffer as a result of Brexit or that Britain has substantial financial obligations to the EU budget that must be fulfilled.

Those obligations are not a “Brexit bill” or an “exit fee,” one senior EU official said, but simply reflect joint budget commitments that the U.K. agreed as a full-fledged EU member.

“That issue will not go away,” the official said. “We’re telling them it will be a problem.”

One EU diplomat, asked how things went at the dinner in London, said: “Badly. Really badly.” The diplomat added: “That’s what I think we have … a possible scenario of great difficulty.”

Pressed on different views of the U.K.’s financial obligations, the diplomat said: “I’m not going to tell you their number, because you are going to laugh.”

The diplomat's overall verdict on the Brits? "They are in a different galaxy."

Another EU diplomat briefed on the dinner said that it was tense, in part because May voiced her opposition to the EU’s insistence on strictly phased negotiations, in which there must be “substantial progress” on settling the terms of withdrawal before turning to the framework of a future relationship.

The U.K. wants the future relationship, anchored by a robust free trade agreement, on the table from the start.

“It didn’t go well,” the diplomat said. “She does not accept the two-phased sequencing. She wants both.”

The diplomat said that May and Juncker each seemed to dig in. “She toughened her tone,” the diplomat said. “So Juncker toughened his.”

Florian Eder and Maïa de la Baume contributed to this article.

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