Britain risks a "disorderly" Brexit after failing to offer up any workable solutions for the Irish border, Michel Barnier has warned.
The EU chief negotiator issued the stark warning after Theresa May's vision for a new customs arrangement was demolished in five rounds of technical negotiations in Brussels.
Sources in the Republic said the Irish government may now have no choice but to stall the talks completely in June.
Officials believe the next 10 weeks must bring "significant and measurable progress" or the entire Brexit timeline will be derailed.
The ultimate deal between the UK and EU is due to be finalised by October 31, but an EU Summit in June is growing in importance.
Mr Barnier said there is still no agreement on 25% of Brexit negotiations, meaning there is a "risk of failure".
"I say as the Union's negotiator that there are still difficulties, still a risk of failure.
"On 25% of the text, we don't have agreement.
"If there is no agreement, there is no orderly withdrawal, there is a disorderly withdrawal and there is no transition," he said.
If the UK crashes out of the EU in March 2019, it may have to remain within the Customs Union in order to keep the Irish border open. However, this would prevent the Prime Minister from striking new free trade deals with non-EU nations.
Asked if the UK could obtain a "single market a la carte" deal, Mr Barnier replied: "No way". The EU has repeatedly said that Britain will not be able to "cherry pick" and enjoy the benefits of the Single Market after Brexit.
DUP MEP Diane Dodds accused Brussels of fresh "bully-boy tactics", saying they were a new low in the Brexit process.
"Following on from Donald Tusk's intervention earlier this week, it is clear that the EU's capacity for compromise and creativity is severely lacking," she said.
"At a critical time for negotiations, there is a need for cool heads and practical minds. Differences should be thrashed out at the negotiating table, not through press headlines."
SDLP MLA Daniel McCrossan accused the DUP and Sinn Fein of failing to protect the interests of all people across the island against Brexit.
Taoiseach Leo Varadkar indicated yesterday that he is prepared to initiate the so-called 'Irish protocol' agreed last December in the event of a disorderly Brexit. "We have a workable and legally enforceable solution that will allow us to avoid a hard border on our island. It's the Irish protocol.
"It's written down in black and white and we believe that provides a solution that allows us to avoid a hard border on our island after the transition period," Mr Varadkar said.