Hungarian Prime Minister Viktor Orban and Irish Prime Minister Leo Varadkar at the Western Balkans summit in Sofia | Virginia Mayo/EPA-EFE Leo Varadkar: No Brexit deal without border backstop Irish prime minister says if the UK has an alternative proposal, the EU needs to see it ‘in black and white.’

SOFIA — It's backstop or bust.

Irish Prime Minister Leo Varadkar warned the U.K. there would be no Brexit deal with the EU27 without a "legally operable" backstop that would prevent a hard border on the island of Ireland.

"We need to have that backstop, because that gives us an assurance that there will be no hard border on our island," Varadkar said, arriving at a summit of EU leaders in the Bulgarian capital of Sofia on Thursday.

The U.K. has been working on alternative proposals after making clear that it will not sign a deal including the backstop provision, as drafted by the European Commission in a draft withdrawal treaty in March, which calls for keeping Northern Ireland inside the EU's customs union. The backstop would come into force if negotiators fail to agree a workable alternative.

While U.K. Prime Minister Theresa May has agreed to the backstop as a general principle, she told the House of Commons in February that the EU's interpretation is unacceptable. “No U.K. prime minister could ever agree to it,” she said. “I will be making it crystal clear to [Commission] President [Jean-Claude] Juncker and others that we will never do so.”

May reiterated her opposition to the backstop as currently interpreted by the EU in meetings on Thursday with Juncker, Tusk and Varadkar.

EU officials have tried to insist that their backstop would remain the fallback in the absence of a more comprehensive solution for the Northern Ireland border. They’ve also said it is the U.K.’s responsibility to devise such a solution.

Varadkar said the EU would consider anything put forward by the U.K. side, but he pointedly warned there have been no signs of any workable proposal and that there could be no withdrawal agreement without a backstop.

In London, officials have been working on an alternative backstop plan that would call for the entire U.K. to remain in a temporary customs arrangement with the EU — specifically by staying within the EU's common external tariff for goods for a time-limited period.

Varadkar suggested that would not be good enough, but he also said the EU and its Task Force 50 negotiating team have seen nothing in writing.

"The Irish position stands," he told reporters arriving at the summit venue in Sofia ahead of a bilateral meeting with May. "We stand by the text of the withdrawal agreement and the Irish protocol, the backstop that was published in March as does the Task Force, as do the 27 member states certainly that are behind us.

Varadkar continued that Dublin "stand[s] by the same position that we have had for a very long time now that there can be no withdrawal agreement without the backstop and we need that backstop to ensure there is no hard border. If the United Kingdom wants to put forward alternatives to that — whether it’s alternative text to the backstop, or whether it’s some sort of alternative future relationship between the U.K. and the EU — we’re willing to examine that.

"But we need to see it written down in black and white," he added. "We need to know that it’s workable and legally operable and we have yet to see anything that remotely approaches that."

An EU official said that negotiations on the backstop would continue. The EU’s chief negotiator, Michel Barnier, has insisted that both the EU and U.K. are committed to a backstop. In a recent speech in Ireland, Barnier said May had confirmed as much in a letter to European Council President Donald Tusk.

“In March, in a letter to the European Council President Donald Tusk, Prime Minister Theresa May confirmed her commitment to including operational legal text on the backstop in the Withdrawal Agreement,” Barnier said. “To be clear: without a backstop, there can be no Withdrawal Agreement. This is an EU issue, not only an Irish issue."

Asked if the U.K. would have to put forward its detailed proposal before the EU leaders summit in Brussels in late June, Varadkar warned that time is running short.

"If we are not making real and substantial process by June then we need to seriously question whether we are going to have a withdrawal agreement at all," he said.

A U.K. government spokesperson said they wanted to give certainty to people and businesses in Northern Ireland and Ireland "as soon as we can."

"We agreed with the EU that a backstop option to avoid a hard border between Northern Ireland and Ireland and protect the constitutional and economic integrity of the U.K. should be translated into legally binding text in the Withdrawal Agreement," the spokesperson said.

"We stand by those commitments but the prime minister has been clear that in its current drafting, the EU's version of the backstop legal text is unacceptable and we continue to negotiate."

UPDATED: This article was updated to reflect the most current comments from U.K. Prime Minister Theresa May and an official U.K. government response.