European Council President Donald Tusk tells Theresa May that EU leaders are unanimous in opposing her Brexit plans.

The so-called Chequers plan would be unacceptable to EU leaders, Tusk tells reporters.

Tusk says the plans being pushed by May would undermine the EU Single Market.

Conservative MPs are threatening to vote down May's proposals.

LONDON — Theresa May's Brexit plan "will not work" the EU's 27 leaders have told her in a major blow to her chances of securing a deal on Britain's future relationship with the EU.

European leaders met on Thursday in Salzburg to discuss the progress of negotiations with the UK. Following their meeting, European Council president Donald Tusk told reporters that the leaders had unanimously agreed that May's plans to stay in a single market for goods, would not be acceptable.

"Everybody shared the view that while there are positive elements in the Chequers proposal, the suggested element for economic cooperation will not work, not least because it risks undermining the single market," Tusk said.

Responding to Tusk's comments, May insisted that her deal "remains the only serious and credible option," on the table.

"I understand referencce has been made to the integrity of the Single Market. Actually we looked at that. We recognised the EU's desire to protect the integrity of the single market and that's why we have put forward a proposal which does exactly that."

However, she said that she would "sit down and listen" to the concerns of EU leaders.

She added: "I believe we can deliver a deal.... that is in the interests of the European Union."

In another blow to May's hopes of winning over EU leaders, French president Emmanuel Macron told reporters that Brexit was a project which had been pursued by "liars."

"Brexit is... pushed by those who predicted easy solutions," he said.

"Those people are liars. They left the next day so they didn’t have to manage it."

While some progress has been made in recent weeks on negotiations about the terms of the Brexit withdrawal agreement, negotiations over Britain's future relationship with the EU remain stuck over May's plan to keep aligned with the EU market in goods, but not in services.

EU leaders have described this as and attempt to cherry-pick the single market, amid fears that it could trigger other member countries to seek to renegotiate the terms of their own EU membership.

The comments come as the prime minister struggles to secure the support of her own MPs for her so-called Chequers proposals.

Hardline Brexit-supporting Conservative MPs are strongly opposed to the proposals and have threatened to vote down any deal along these lines.

The prime minister suffered a series of senior resignations from her Cabinet over the summer on the issue.