Speaking in 10 Downing Street a day after the humiliating rejection of her Brexit plans at the European Union summit in Salzburg, the Prime Minister recognised that negotiations had reached an "impasse" with just six months to go.
In a hastily-arranged event inside Number 10 this afternoon, a defiant display from the Prime Minister saw her hit back after yesterday's summit.
Mrs May says her plan will prevent a hard Irish border, and mean no divergence between Northern Ireland and mainland Britain.
At her own end-of-summit news conference, May doubled down on her Chequers plan for Brexit, saying it "remains the only serious and credible proposition on the table".
May's proposals - dubbed the Chequers Plan after the Prime Ministerial retreat at which they were forged over the summer - which would keep Britain closely aligned to the EU's single market on agriculture and goods (but not services) via a common rule book "will not work" because it risked undermining the single market.
But Brussels negotiators were at pains to stress a continued willingness to conclude a treaty and are looking for a spirit of mutual compromise after October 3, when May will, they believe, have survived her Conservative party's conference despite attacks on her plans to keep British trade close to the EU.
In her statement Friday, May said she would "never agree" to "any form of customs border between Northern Ireland and the rest of the U.K". The consequences could be dire for Brexit and, although May is 1/4 to still be PM by April 1 next year, there could be implications for her premiership, as she now looks weakened in Brussels and at home.
If there is no divorce agreement, there will be no transition - a two-year grace period created to prevent the country and its businesses from tumbling into a legal limbo.
Theresa May insisted that her plan was the only one on the table and that Britain was prepared to walk away from the European Union without a deal if it was rejected.
Theresa May puts ball back in EU's court amid Brexit impasse
"Throughout this process I have treated the European Union with nothing but respect, I expect the same of EU". I will not overturn the result of the referendum.
However, Mr Tusk said the Prime Minister was "surprisingly tough" and "uncompromising" during negotiations at this week's EU Summit in Salzburg.
"It is not acceptable to simply reject the other side's proposal without a detailed explanation and counter-proposals", May said in a televised statement from Downing Street. May had a chance to sell her plan over dinner on Wednesday but leaders were unimpressed, with some saying she had told them nothing new.
"While both sides want a deal, we have to face up to the fact that despite the progress we have made there are two big issues where we remain a long way apart".
Another diplomat present said: "I think she came to Salzburg with false expectations, probably caught up in what was mainly the UK's own spin about the 27 getting weak and divided".
"Anything which fails to respect the referendum or which effectively divides our country in two would be a bad deal and I have always said no deal is better than a bad deal", she said.
Stephen Crabb said he was "taken by surprise" at the way they dismissed her Chequers plan for future relations. To deny its legitimacy, or frustrate its result, threatens public trust our democracy.
An additional blow came from French President Emmanuel Macron, who branded Brexiteers as "liars".