Michel Barnier insisted that the EU had no choice but to begin drafting such legal language | Olivier Hoslet/EPA EU issues Brexit ultimatum on Ireland EU will draft legal text declaring Northern Ireland will remain in customs union.

The EU will prepare a draft of the U.K. withdrawal treaty that envisions Northern Ireland remaining in the customs union — essentially issuing an ultimatum that London come up with other options or accept that there is no other practical way to avoid the recreation of a border between Ireland and Northern Ireland, the EU's chief negotiator, Michel Barnier, said Friday.

Speaking at a news conference alone, rather than alongside his U.K. counterpart, David Davis, Barnier insisted that the EU has no choice but to begin drafting such legal language because London has offered no clarity on how the Ireland-Brexit conundrum might be solved as part of an overall new relationship.

"It's important to tell the truth," Barnier said, suggesting that British officials have not been forthright about the implications of their public pronouncements. "A U.K. decision to leave the single market and leave the customs union would make border checks unavoidable."

Barnier noted that in the "joint report" that concluded Phase 1 of the negotiations in December, the two sides had allowed for three options on Ireland: a solution through an overall new relationship; alternative solutions put forward by the U.K., that would avoid reinstating a border; and maintaining all existing regulatory rules and procedures — effectively keeping Northern Ireland in the EU's customs union.

"The third option is to maintain full regulatory alignment with those rules of the single market or the customs union, current or future" — Michel Barnier

Barnier on Friday said an update on the future relationship from the U.K., which had been scheduled for Friday, had been cancelled by the British. On the second option, he said, "We are waiting for such solutions."

U.K. officials claimed that the future relationship “update” meeting had not been cancelled and was taking place Friday afternoon. Barnier said later in his press conference that EU negotiation coordinator Sabine Weyand would be meeting U.K. officials later Friday, but added “it wasn’t possible for [a] presentation” on the U.K.’s “choices” about the future relationship to be given.

On Ireland, he said: "The third option is to maintain full regulatory alignment with those rules of the single market or the customs union, current or future," adding: "In the meantime, it is our responsibility to include the third option in the text of the withdrawal agreement, to guarantee there will be no hard border, whatever the circumstances. This means we must start legally defining how the scenario would work in operational terms."

"There must be no ambiguity here," he said.

Barnier insisted that the U.K. side “had accepted the necessity” of preparing for how the current regulatory framework would be maintained on the island of Ireland “provided we discuss the other two options in parallel and that is what we will work on in the coming rounds.”

But the prospect of maintaining the EU’s full customs regime is likely to be a non-starter among ardent Brexiteers and also many in Northern Ireland, including the Northern Irish Democratic Unionist Party, whose 10 MPs guarantee Prime Minister Theresa May’s majority in the U.K. parliament.

May insisted on language in the December “joint report” making clear that Northern Ireland would not be treated differently in Brexit than the rest of the U.K. Maintaining the customs union, however, would either require all of the U.K. to stay within the existing EU trade framework, or would require a customs border between Northern Ireland and the rest of the U.K.

Barnier repeated his assertion, first made after a meeting with Davis in London on Monday, that the time had come for the U.K. "to make a choice."

Barnier on Friday also expressed bewilderment at the U.K.'s objections to the terms of a transition period proposed by the EU, and suggested that the British side was creating a risk that there might not be an agreement on transition at all. That would potentially subject the U.K. economy to a severe shock on the official withdrawal date of March 29, 2019, as officials broadly agree that the British government will not be ready with its contingency plans by then.

Charlie Cooper contributed reporting.