The U.K. hasn’t put forward a plausible plan to prevent the emergence of a hard Irish border after Brexit, risking the country crashing out of the European Union without a deal, Irish Prime Minister Leo Varadkar said.

“If we are not making real and substantial progress by June then we need to seriously question whether we are going to have a withdrawal agreement at all,” Varadkar told reporters on the way into a summit on wider EU issues in Sofia.

U.K.-EU negotiations are scheduled to resume in Brussels next week, ahead of a summit in June. U.K. Prime Minister Theresa May is preparing a plan to solve the Irish border problem by keeping some EU customs rules for years after Brexit, according to people familiar with the matter. The plan, met with skepticism among EU officials, would keep the U.K. aligned with tariff and customs rules beyond the planned 21-month transition period.

The EU insists that if the overall Brexit treaty, including the transition period, is to be signed, it must include a “backstop” to keep the Irish border invisible. May has said the EU’s idea, to keep Northern Ireland in the EU’s customs union and parts of the single market while the rest of the U.K. goes its own way, is unacceptable.

But Varadkar, who is scheduled to hold a private meeting with May on the sidelines of the Sofia summit on Thursday, said he hasn’t seen any credible alternative U.K. plan.

The EU must see any U.K. idea “written down in black and white, and we need to know that it’s workable and legally operable and we’ve yet to see anything that remotely approaches that,” he said.