THE talks on the UK’s post-Brexit relationship with the EU are to go to the wire as Theresa May will return to Brussels on Saturday just hours before a special summit is due to take place on Sunday.

After a 90-minute meeting in the Belgian capital with Jean-Claude Juncker, the President of the European Commission, the Prime Minister emerged to say that “further progress” had been made.

“As a result we have given sufficient direction to our negotiators, I hope, for them to be able to resolve the remaining issues. And that work will start immediately,” explained Mrs May.

She added: "I now plan to return for further meetings, including with President Juncker on Saturday to discuss how we can bring to a conclusion this process and bring it to a conclusion in the interests of all our people."

Earlier, doubt was cast on whether or not the Sunday summit would even take place after Germany’s Angela Merkel threatened a boycott because of the row over member states’ continuing demands from fishing rights to Gibraltar.

Valdis Dombrovskis, the Commission Vice-President, said that for Sunday's summit to go ahead, "we will need to have agreed beforehand the political declaration on the future relationship and we are not there yet".

Asked whether the UK expected the summit to proceed, the PM's spokesman said only: "A summit has been called, an agenda has been published and we look forward to attending."

Earlier at Westminster, the PM faced calls from Tory MPs to change tack with one backbencher warning her proposed deal threatened the integrity of the Union.

But Mrs May said if Brexiteer Tories voted down her Brexit plan next month, they risked “no Brexit at all”.

While earlier in the week, David Mundell, the Scottish Secretary, made clear there was only one choice before MPs - Mrs May’s deal or no-deal - his Cabinet colleague Amber Rudd gave a different take, saying: “The Commons will stop no-deal. There isn't a majority…to allow that to take place."

The new Work and Pensions Secretary said she expected MPs to back the PM’s plan after peering into the "abyss" and pulling back.

But she added: "If it doesn't get through, anything could happen; the Brexiteers may lose their Brexit."

Chancellor Philip Hammond told ITV's Peston: "It's clear that if the deal is not approved by Parliament we will have a politically chaotic situation. And, we don't know what the outcome of that will be."

During PMQs, Mrs May clashed with Jeremy Corbyn, who branded her Brexit plan a “failure,” saying: "Instead of giving confidence to the millions of people who voted both Leave and Remain, this half-baked deal fails to give any hope that it can bring the country together again.

"Isn't it the case that Parliament will rightly reject this bad deal and if the Government can't negotiate an alternative then it should make way for those who can and will."

The PM hit back, saying: "He is opposing a deal he hasn't read, he's promising a deal he can't negotiate, he's telling Leave voters one thing and Remain voters another - whatever he will do, I will act in the national interest."

Later, Ian Blackford, the SNP leader at Westminster, accused Mrs May’s Government of being in “chaos and confusion” and said it was extraordinary the SNP was able to get its amendments to the Finance Bill through without a vote because the Democratic Unionists had failed to supply support to the Conservatives.

“It shows the Government is running scared and there is a sense of panic,” declared the Highland MP.

Mr Corbyn’s spokesman accused the Government of being “in disarray,” saying: “It’s clear it’s unable to deliver a deal that has the confidence of Parliament or the country.”

Elsewhere, Mrs May got a boost when NFU Scotland came out in favour of her withdrawal plan.

Andrew McCornick, the body’s President, said while the PM’s plan was “not perfect,” it would ensure there were “no hard barriers on the day we leave the EU and will allow trade in agricultural goods and UK food and drink to continue throughout the transition period largely as before”.

A UK Government spokesman welcomed the union’s support and urged the Scottish Government “to do the same,” adding: “The deal will allow for the UK’s smooth and orderly exit from the EU and it works for the whole of the UK, including Scotland.”