LISBON — EU negotiator Michel Barnier urged Britain on Saturday to stop playing “hide and seek” in Brexit talks as the two sides traded blame over the lack of progress in negotiations.

Barnier delivered his warning in a speech just days after the EU dismissed London’s latest Brexit plans as “fantasy” following a round of technical talks in Brussels.

British ministers said the barbs were “not helpful” as pressure rises to make progress on the key issue of the future of the Irish border by a June 28-29 EU summit.

“To negotiate effectively, you need to know what the other party wants. A negotiation cannot be a game of hide and seek,” Barnier told an audience of EU law experts in the Portuguese capital.

Barnier said that Britain must “look at the reality of the EU in the face, and also the reality of Brexit in the face” and stop making unreasonable demands regarding its future relations with Europe.

The former French minister underlined that Britain had chosen to leave the EU and that recent proposals by London to remain part of certain EU institutions or agencies after Brexit were non-starters.

Barnier said the EU also refused to take responsibility for any bad fallout of Brexit.

“I can see the temptation of the blame game, which would consist in bringing the negative consequences of Brexit on the European Union.

“But we will not be impressed. I won’t be impressed,” he said.

He chided British criticism of EU positions as a “blame game”, urging London to recognize that it could not retain many elements of EU membership after Brexit.

Referring to discussions within Prime Minister Theresa May’s government on whether to drop an insistence on having no customs union, he said: “If the United Kingdom would like to change its own red lines, it must tell us. The sooner the better.”

“We are asking for clarity,” he added. “A negotiation cannot be a game of hide and seek.”

On the issue of the governance of a withdrawal treaty, which both sides hope to have ready around October, Barnier repeated the EU’s insistence that primacy of the European Court of Justice inside the Union be maintained in regulating any dispute that could not be resolved by a joint committee appointed by the political leadership of the two sides.

“We cannot accept that a jurisdiction other that the Court of Justice of the European Union determines the law and imposes its interpretation on the institutions of the Union,” he said.

The role of British judges would be respected, he added.

But without an agreement on this, the whole deal would collapse: “Without an agreement on governance, there will be no withdrawal agreement and so no transition period.”

Many businesses are counting on an interim accord to maintain a broad status quo between Britain and the EU after Brexit until the end of 2020.

Barnier, who has been hoping to making substantial progress on key issues before May meets fellow EU leaders at a Brussels summit in a month, also criticized what he called a “blame game” in which British officials were accusing the EU of failing to show flexibility to allow continued close cooperation in areas such as security, the economy and research.

This, Barnier said, was to ignore the close legal framework within the EU which was the basis for trust and cooperation among its nation-state members. “We cannot share this decision-making autonomy with a third country,” he said.

“The United Kingdom must face up to the reality of the Union ... It is one thing to be inside the Union and another to be on the outside.” — Agencies