THE Brexit talks have broken down over UK Government concessions on the Irish border after the Democratic Unionist Party intervened to block a deal which it branded a “Unionist nightmare”.

The DUP’s anger came after Theresa May drew up a draft plan to agree “regulatory alignment” on trade between Northern Ireland and the Republic; regarded as effectively keeping the UK region in the single market and customs union.

When the news broke, Nicola Sturgeon swiftly intervened to insist that if Northern Ireland could get a special deal, so too should Scotland: Sadiq Khan, the London mayor, and Carwyn Jones, the First Minister of Wales, made similar appeals for London and Wales respectively.

Ms Sturgeon argued keeping Northern Ireland effectively in the single market made it “all the more vital for Scotland’s national and economic interests that we are too”.

She added: “Today’s developments also demonstrate in the clearest possible way, through the role played by the Irish Government at the top table in Europe, the importance of being independent when it comes to defending your vital national interests.”

But John Lamont for the Scottish Conservatives decried the FM for trying to “weaponise Brexit,” insisting it was not in Scotland’s interests to see the UK’s internal market disrupted.

“If Nicola Sturgeon is trying to further her constitutional goal by creating different regimes on either side of the border, then she is selling Scotland short,” claimed the Tory MP.

For several hours a leak about the draft UK-EU agreement dominated the headlines as the Prime Minister sat down for a long lunch in Brussels with Jean-Claude Juncker, hoping the European Commission President would agree that “sufficient progress” had been made on the divorce bill, citizens’ rights and the Irish border to move the talks onto trade and transition.

After a frenetic weekend of diplomacy between London, Brussels and Dublin, the mood in the Irish capital lifted considerably on Monday morning with Simon Coveney, the Deputy Prime Minister, declaring: “The legitimate concerns that Ireland has been raising for months are going to be addressed fully.”

But once Arlene Foster, the DUP leader, got wind of the draft deal and the reference to “regulatory alignment,” she quickly issued a statement at Stormont that dramatically put the brake on the talks.

She insisted her party – whose 10 MPs secure the Conservative Government’s Commons majority - could not accept any divergence on trade rules, which separated Northern Ireland economically or politically from the rest of the country.

“The economic and constitutional integrity of the United Kingdom must not be compromised in any way,” she insisted.

Her colleague, Sammy Wilson, noted if the PM “gives in on special demands for Northern Ireland, then she will be giving in on special demands for Scotland and other parts of the United Kingdom; it's a Unionist nightmare".

Tory backbencher Anna Soubry, the former Business Minister, noted how Mrs May’s offer had been “a gift for the SNP,” noting: “The Nationalists will want the same thing; it’s madness.”

Nigel Farage, the former Ukip leader, called on Mrs May to "leave office now", branding proposals for regulatory alignment in Ireland a “bitter betrayal".

In Brussels, Mrs May had to break away from her power lunch with Mr Juncker to take a phonecall from Ms Foster. Minutes later, and much to the PM’s embarrassment, she had to tell her host that the draft deal was off.

At a subdued and short press conference, Mrs May said “further negotiation and consultation” was needed but she remained confident a breakthrough to take the talks to phase two would materialise.

Mr Juncker, who described his meeting with the PM as "friendly and constructive,” told reporters: “I have to say we were narrowing our positions to a huge extent today, thanks to the British Prime Minister, thanks to the willingness of the European Commission to have a fair deal with Britain.

"I'm still confident that we can reach sufficient progress before the European Council of December 15. This is not a failure, this is the start of the very last round. I'm very confident that we will reach an agreement in the course of this week," he added.

Mrs May could return to Brussels later this week for more talks; the crunch European Council, when a decision by the EU27 has to be made, takes place on December 14 and 15.

In Dublin, Leo Varadkar, the Taoiseach, could not hide his displeasure at the turn of events, saying: “I’m surprised and disappointed the British Government now appears not to be in a position to conclude what was agreed earlier today.”

At Westminster, Jeremy Corbyn insisted the real reason for Mrs May’s failure to secure a breakthrough in the Brexit talks was the “grubby deal the Government did with the DUP after the election”.

The Labour leader said “after months of delays and grandstanding,” it was disappointing no progress had been made in the Brexit negotiations.

“Each passing day provides further evidence that Theresa May’s Government is completely ill-equipped to negotiate a successful Brexit deal for our country,” added Mr Corbyn.

Stephen Gethins for the SNP said: “Reports that the DUP sunk today’s deal are incredibly worrying; it's a further example of the Tory/DUP coalition of chaos. The DUP can’t be permitted to run the show.”

Tom Brake, the Liberal Democrat Brexit spokesman, added: "This whole situation could have been solved by keeping the entire country in the single market and the customs union. Instead, Theresa May has cowered in front of her backbenchers and driven forward a reckless Brexit, which risks destabilising the whole UK.”