(CNN) British Prime Minister Theresa May has told the European Union that the UK will not accept its proposals to deal with one of the thorniest issues in Brexit talks -- the issue of the Irish border.
In a speech in Belfast Friday, May criticized the EU's "backstop" plan, whereby Northern Ireland would remain closely tied to European regulations after Britain leaves the bloc, in the event of the two sides failing to reach an agreement on a future relationship.
The proposal is intended to avoid a "hard" border between Northern Ireland, which will leave the EU as part of the UK, and the Republic of Ireland, which remains in the bloc. Border infrastructure such as customs and security posts were removed as part of the Good Friday Agreement that brought peace to Northern Ireland in the late 1990s.
But, if implemented, it would effectively create a border down the Irish Sea between the island of Ireland and Great Britain. May said that would be unacceptable to any British prime minister. May has proposed instead that any backstop should apply to the whole of the UK.
"The economic and constitutional dislocation of a formal 'third country' customs border within our own country is something I will never accept and I believe no British prime minister could ever accept," May said in her speech at Belfast's Waterfront Hall.
Referring to a White Paper on Brexit published by the government last week, May said London had laid out its position and now it was time "for the EU to respond."
The border issue is politically perilous for May. The Democratic Unionist Party -- the right-wing Northern Irish party which is currently propping up May's minority Conservative government in Westminster -- has vowed not to accept any suggestion that Northern Ireland would have a different relationship with the EU than the rest of the UK.
At the same time, the reappearance of infrastructure on the Irish border would significantly increase the risk of sectarian violence.
May's hardline stance on the issue will also raise fears that Britain and the EU will fail to reach a deal on their future relationship before Britain is due to leave the EU on March 29 next year.
The EU is already stepping up its preparations for a "no-deal" scenario. In a communique to member states published Thursday, the European Commission warned "there is no certainty that an agreement will be reached."
It urged the remaining 27 member states to prepare for a chaotic Brexit, noting that customs arrangements, airline agreements and arrangements for financial transactions could end suddenly without any new plans being put in place.
"Contingency planning for the worst possible outcome is not a sign of mistrust in the negotiations," it said. "The Commission is devoting very significant resources and committing great efforts to achieve an agreement. This remains our goal. However, the outcome of negotiations cannot be predicted."
Britain is also making preparations for a no-deal Brexit. May told MPs on Thursday that the British government would publish around 70 "technical notices" in the next few months that will tell business and citizens what they should do in the event of Britain leaving the EU without a deal.
May denied suggestions that a no-deal Brexit was more likely. "If we are in a no-deal scenario then we will lay out the consequences for the public. What we are doing at the moment is working for a deal," she told members of the House of Commons liaison committee.
Speaking on Thursday the EU's chief negotiator, Michel Barnier, said it was "a matter of urgency to agree on legally operative backstop for Ireland and Northern Ireland (as) an all-weather insurance policy."
"Our challenge will be to find common ground between the fundamental principles that define the EU, and the UK positions," he added.
Following a meeting with Barnier, new UK Brexit secretary Dominic Raab rejected suggestions the UK's position "hangs by a thread," and said "we're getting ready at home for the domestic preparations that we're making."
"We've only got 12 weeks really left to nail down the details of the agreement," he added. "I think as you get closer to the line, the preparations need to be intensified."
May's defiant challenge to EU leaders belies the weakness of her position. She only narrowly avoided defeat on a key Brexit bill this week, as rifts widened within her Conservative Party between pro-EU and pro-Brexit MPs.
The pro-EU faction was furious that, earlier in the week, May's government had made a number of concessions to the Brexit-supporting faction, and introduced an amendment to a Brexit bill that was only defeated by six votes, several of which came from the opposition benches.
Earlier this month two of May's top Cabinet ministers -- Brexit secretary David Davis and Foreign Secretary Boris Johnson -- quit over what they see as a watering-down of the UK's blueprint for Brexit.
While May moved quickly to replace both men, she also accepted a number of key demands by hardline anti-EU Conservatives which many saw as undermining the original Chequers agreement -- a plan for a Brexit strategy hatched at May's official country residence 10 days ago.