Britain is set to unveil new proposals on the future status of the Northern Irish border, in a bid to break deadlock in Brexit negotiations.

Speaking at the end of a two-day EU summit in Austria, at which Brexit was high on the agenda, Theresa May said the UK would “shortly” be coming forward with new proposals on the so-called “backstop” arrangements for implementation at the border if no long-term solution is found.

Her announcement came as European Council president Donald Tusk told the Prime Minister she will have to rethink her Chequers blueprint for Brexit, saying the plan will not work in its present form.

Following a working lunch of the remaining 27 EU states at the informal summit in Salzburg, Mr Tusk said that while there were “positive elements” in the Chequers plan, they could not accept any proposal which threatened the single market.

European Council president Donald Tusk warned that Theresa May’s Chequers plan will not work (European Commission Audiovisual Services/PA)

“Everybody shared the view that while there are positive elements in the Chequers proposal, the suggested framework for economic co-operation will not work, not least because it is undermining the single market,” he said.

German Chancellor Angela Merkel said that the EU27 were agreed that, “in the matter of the single market, there can be no compromises”.

After what she described as a “frank” meeting with Mr Tusk, Mrs May – who was not present at the lunch – insisted that the plan drawn up at her country residence in July remains “the only serious and credible proposition on the table” for resolving the issue of the Irish border.

She has flatly rejected a European Commission backstop proposal for Northern Ireland to remain within the EU customs area after Brexit, arguing that this would draw a border down the Irish Sea.

Speaking after her meeting with Mr Tusk, the PM said: “We both agree there can be no withdrawal agreement without a legally-operative backstop. But that backstop cannot divide the United Kingdom into two customs territories, and we will be bringing forward our own proposals shortly.

“On the economic partnership, there is no solution that will resolve the Northern Ireland border which is not based on the frictionless movement of goods.

“Our White Paper remains the only serious and credible proposition on the table for achieving that objective.”

Mrs May acknowledged there was “a lot of hard work to be done” to secure agreement on the withdrawal agreement and a political declaration on future trading relations.

But she said: “If the political will is there on the other side, I’m confident we will reach a deal and to do so is in the EU’s interests as well as the UK’s.”

Mr Tusk said the “moment of truth” in the negotiations would now come at the next full European Council meeting in October, when it will be decided whether to hold a special summit in November to finalise the withdrawal agreement.

“In October we expect maximum progress and results in the Brexit talks and then we will decide whether conditions are there to call an extra summit in November to finalise and formalise the deal,” he said.

He added: “Today I am a little more optimistic when it comes to a positive outcome of our negotiations.

“Unfortunately we cannot at this stage exclude a no-deal – it depends on both sides of negotiations.”