NORTHERN Ireland’s Democratic Unionists are threatening to vote down the Autumn Budget if Theresa May breaches its “blood red line” on the Irish backstop in the Brexit talks.

The threat emerged at a background briefing for broadcasters and is borne out of fears that some form of new regulatory checks between Northern Ireland and the rest of the UK could be agreed as talks between Britain and the EU enter their most crucial phase.

One source told the BBC: "If Theresa May doesn't take our concerns on board, we will take the view that she is not the leader to take us through to a safe Brexit."

Thus far, the DUP - whose 10 MPs prop up the Conservative minority Government - has failed to spell out what would happen if their red line on no new customs checks was breached. But it has been clear that it would oppose anything that separated off Northern Ireland from the rest of the UK.

Indeed, this has been the Prime Minister’s own red line; that there should be no border down the Irish Sea.

Reports coming out of Brussels suggest an outline deal on the Irish backstop has been agreed but would not involve new checks; rather, an increase in existing ones, which are carried out on food and livestock going from the British mainland to Northern Ireland.

If the DUP were to vote down the Budget, this would lead to Labour tabling a no-confidence vote in the Tory Government. If Mrs May lost this, then there would almost certainly be a general election but this prospect could concentrate MPs’ minds, meaning the PM could win the vote but be badly damaged.

The DUP’s threat to vote down the Budget came as a Downing Street source was asked if Mrs May felt she could rely on the votes of Arlene Foster and her parliamentary colleagues.

"The confidence and supply arrangement we have with the DUP is a matter of record," declared the source. Asked if this was a yes, he replied: "It’s an answer."

The developments came after the PM urged MPs across the Commons to act in the “national interest” and back a Brexit deal amid warnings that "decisive" progress was needed in the negotiations before a crunch Brussels summit next week.

Michel Barnier, the EU's chief negotiator, briefed top Brussels officials behind closed doors at a regular meeting of the College of Commissioners, which also heard Martin Selmayr, the European Commission Secretary General, warn of the need to be prepared for "all outcomes" from Brexit.

During PMQs, Mrs May urged MPs to do their "duty" to implement the result of the Brexit referendum as she struggled to maintain discipline within the Tory ranks.

It was her first appearance in the Commons since last month's Salzburg summit, when EU leaders, amid an acrimonious atmosphere, told her that her Chequers blueprint for Brexit would not work.

The PM has come under growing pressure from Brexit-backing Tories to drop the plan agreed by the Cabinet at her country residence in July and instead seek a Canada-style free trade deal.

But veteran Europhile Tory Kenneth Clarke urged her to ignore hardline Brexiteers and bring forward a deal acceptable to pro-EU MPs on both sides of the Commons.

The former Chancellor said that would reveal that the "hardline Eurosceptic views" of the "Bennites" in the Labour leadership and the "right-wing Nationalists" on the Tory benches were in the minority in the Commons.

Labour has indicated it will oppose a deal based on the Chequers Plan and will push for a general election or, possibly, a second referendum.

Eurosceptic Tories claim dozens of Conservatives are also prepared to vote against a Chequers-style deal.

Mrs May told MPs: "I would hope that everybody across this whole House will put the national interest first."

She added that they should also remember that Parliament gave the British people the decision over Brexit and following the 2016 referendum result "it is our duty to ensure that we leave".

In other developments:

*John Glen, the Treasury Minister, backed a previous prediction by the Bank of England that around 5,000 financial services jobs would have been shifted out of London by "Day One" on March 29;

*Sir Amyas Morse, who heads the spending watchdog, the National Audit Office, said it was "not impossible" that flights would be grounded if there was an "unfriendly" break-up with Brussels and

*the International Monetary Fund said "growing anxiety" about a breakdown in Brexit negotiations could result in "contractual and operational uncertainties in the United Kingdom and elsewhere in Europe".

The negotiations are now reaching a critical stage ahead of next week's European Council summit.

The October 18 meeting was initially framed as the deadline for agreement on the terms of UK withdrawal from the EU, as well as the publication of a political declaration on future relations in areas such as trade and security.

But with both sides confirming that differences remain on issues such as the status of the Irish border, expectations are growing that the final moment of decision will be put back to a special summit in November.

Following the College of Commissioners meeting, two commissioners stressed the importance of continued UK/EU co-operation on security after Brexit.

Julian King, the British Security Commissioner, said: "There are some issues that will need to be addressed in the context of a future partnership, including on security. There is a strong shared self-interest in finding a way of maintaining co-operation on security, if that proves to be possible."

Dimitris Avramopoulos, the Home Affairs Commissioner, said: "I don't know what will be the outcome of these negotiations. Let's hope that we are going to have a positive end.

"But as far as security is concerned, it is very important in order to ensure the security of our citizens and safety for our countries, to keep working together in the future."

Reports had suggested that Mr Barnier would present a paper setting out new EU proposals at Wednesday's meeting but it is thought that this has been delayed until closer to the summit date.

The Government is also due to publish its own revised proposals for the Irish border but Downing Street has said only that these will appear "in due course".

Speaking after Mr Barnier's briefing, Margaritis Schinas, the Commission spokesman, told reporters: "He recalled that decisive progress must be made in time for the October European Council next week. Negotiations at technical level will continue this week."

Mr Selmayr provided an update on preparations for Brexit, and reminded commissioners of "the importance for all stakeholders to prepare for all outcomes at all levels", explained Mr Schinas.

His comments came after it emerged that the UK Government is recruiting a team of civil emergency workers earning up to £50,000 to help the country cope with any disruption caused by Brexit.

David Davis, the former Brexit Secretary, warned on Tuesday of "dire" consequences for Conservatives at the next general election if the Government sticks to its negotiating stance on EU withdrawal.

In a letter to fellow Tory MPs, he said a deal based on Mrs May's Chequers plan would deliver "none of the benefits of Brexit" and reduce the UK to being "a rule-taker from Brussels".