THERESA May has urged Brussels to drop its “unacceptable” demands as she prepares to sell her compromise plan to her fellow EU leaders over dinner at an informal summit in Salzburg this evening.

After Michel Barnier, the EU’s chief negotiator, suggested Brussels was preparing to make an “improved offer” to unblock the logjam on the issue of the Irish border, the Prime Minister used an article in the German newspaper Die Welt to hit back at criticisms of her Chequers Plan.

She wrote: “There have been arguments made against our proposals that have been at odds with the reality of trade negotiations elsewhere and indeed the current trading relationship between EU member states.”

Commenting on the EU’s original proposal – to keep Northern Ireland in the single market and customs union, thus moving the border to the Irish Sea – Mrs May declared: “Neither side can demand the unacceptable of the other, such as an external customs border between different parts of the United Kingdom – which no other country would accept if they were in the same situation – or the UK seeking the rights of EU membership without the obligations.”

Tonight’s dinner is expected to last at least two hours and will take place at the Felsenreitschule, the theatre where the Von Trapp family performed before fleeing the Nazis in The Sound Of Music movie, leading some to quip – the hills will be alive with the sound of Brexit. Tomorrow, over lunch EU leaders will again discuss Brexit albeit without the PM.

Tonight, the PM is expected to have face-to-face talks with Charles Michel, the Belgian premier, with further bilaterals with Leo Varadkar, the Taoiseach, and Donald Tusk, the European Council President, tomorrow.

Mrs May is likely also to talk to other leaders in the margins of the summit in an effort to win backing for a plan which has met fierce resistance from within the ranks of her own Conservative Party.

Her message will be stark: if the EU does not show flexibility and the only deal she can take back to Westminster in November is an unsellable fudge, then MPs will vote it down and there will be a damaging no-deal.

In referring to the EU27’s improved offer, Mr Barnier also made clear that the “moment of truth” would come next month at the European Council. By then, he argued, it should be clear whether or not a deal can be done.

However, Brussels-watchers point out that there is a habit of the European Commission going to the brink and the Salzburg summiteers are expected to announce a special Council meeting in November, probably on the 13th and 14th, to formally seal a deal.

In an interview with the Daily Express today Mrs May said hers was the "right plan" for the UK while representing a "good deal for the EU".

She insisted: "I'm not going to be pushed away from doing what is necessary to get the right deal for Britain."

In her Die Welt article, the PM argues that the Chequers blueprint - a "common rulebook" for trade in goods and "business-friendly facilitated customs arrangement" - is the only way to resolve the thorny issue of the border between Northern Ireland and Ireland.

"It is profoundly in both sides' economic interest, it respects the integrity of the single market and, crucially, no one else has come up with a proposal that could command cross-community support in Northern Ireland that is the only true foundation for stability there."

She adds: “To come to a successful conclusion, just as the UK has evolved its position, the EU will need to do the same.”

Ahead of today’s summit, Mr Tusk wrote to EU leaders said EU leaders should discuss arrangements for the "final phase" of the Brexit talks "including the possibility of calling another European Council in November".

He impressed on them the need for "limiting the damage" caused by Brexit and added: “Unfortunately, a no-deal scenario is still quite possible. But if we all act responsibly, we can avoid a catastrophe."

Meanwhile, David Davis, the former Brexit Secretary, said he expected just "warm words" from the EU leaders in Salzburg, stressing “they won't give much".

He asserted Mrs May’s ultimatum of her deal or no deal was a false choice and predicted the EU would "pile on" extra demands on money, migration and other issues.

"All these things will come back and we'll see more and more pressure, and she will have a deal she won't be able to bring back to the House of Commons because it'll be lumbered with loads of other EU demands, so she's going to have to have something else," he told BBC Radio’s Today programme.

He said a "variety of options" existed and a chance to "reset" would emerge in November.

“We get to a point where she will not be able to accept what they offer, they will not be able to accept what she offers, and there'll have to have some sort of reset. The reset is to step back to what Donald Tusk offered in March…a free trade agreement."

On Northern Ireland, Mr Davis added: "That's one area where the EU is softening."

The Yorkshire MP noted how he had repeatedly told Mrs May there would be a "pressure point" late in negotiations.

"[It’s] always the case with the European Union…they move at the last possible minute; after they've tested your mettle, after they've taken you to the cliff-edge and that's what will happen."