Theresa May is facing a fresh challenge to her Brexit plans from her Democratic Unionist Party allies, who are threatening to vote against the Budget if she breaks their red lines.

The radical move is one of the options being considered by the DUP if attempts to nail down a deal with Brussels include any proposals that would leave Northern Ireland being treated differently to the rest of the UK, the Press Association understands.

The DUP agreed a “confidence and supply” deal to support Mrs May after last year’s general election and losing the party’s support in the Commons would mean possible defeat on the budget at the end of October and a no-confidence vote.

In a move seen by some as a warning shot, DUP MPs failed to back the Government in voting against a Labour amendment to an Agriculture Bill outlining post-Brexit reforms on Wednesday night. Despite their abstention, it was still defeated by 59 votes.

The development came as former foreign secretary Boris Johnson warned that a “backstop” arrangement for Northern Ireland being negotiated by the Government would leave the UK “a permanent EU colony”.

In a series of tweets, Mr Johnson said that the deal would keep the UK in the customs union and Northern Ireland in the single market, and would mean increased checks on goods travelling between the province and the British mainland.

DUP leader Arlene Foster insisted on Tuesday that her party would not accept customs or regulatory checks on goods travelling in either direction between Northern Ireland and Great Britain after meeting EU chief negotiator Michel Barnier in Brussels, and again following talks on Wednesday with European Parliament Brexit co-ordinator Guy Verhofstadt.

The party’s Brexit spokesman Sammy Wilson later explicitly warned the Government against doing a deal that kept Northern Ireland in the Single Market, writing in the Telegraph the party could not support “any deal which includes such economically and constitutionally damaging arrangements”.

He added: “When we entered into the confidence and supply arrangement with the Government, we did so to enable it to deliver on the fiscal elements of its domestic programme and the referendum result to leave the EU.

“If the Government decides in the face of EU belligerence to cut and run and leave part of the UK languishing in the stifling embrace of the EU, then that would be totally unacceptable to us and many others in the House of Commons. It would have implications not just for Brexit legislation – 50% of which would not have passed without DUP support – but also for the Budget, welfare reform and other domestic legislation.

Mr Barnier told a business audience in Brussels that agreement on a withdrawal deal was “within reach” at the crunch October 17-18 summit of the European Council, with “80-85%” of the accord now finalised.

But he made clear that the EU envisages a deal involving new customs and regulatory checks on goods travelling from Britain to Northern Ireland, including health and sanitary inspections for 100% of animals and animal products arriving in ports like Larne and Belfast from the rest of the UK.

The DUP has 10 MPs, although Ian Paisley cannot vote in the Commons until November 20 after being suspended for failing to declare two family holidays paid for by the Sri Lankan government.

Theresa May urged MPs to act in the national interest over Brexit at Prime Minister's Questions (PA Wire/PA)Theresa May urged MPs to act in the national interest over Brexit at Prime Minister’s Questions (PA)

Downing Street insisted that defeat on the budget would not amount to a vote of no confidence in the Government under the terms of the legislation which provides for fixed-term, five-year parliaments.

Senior Cabinet ministers will gather in Downing Street on Thursday for the latest in a series of briefings on the state of negotiations, though it is not expected that any fresh decisions will be made at the meeting. The Government has said it will publish revised proposals for the Irish border “in due course”.

The Prime Minister 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.

Cabinet Office Minister David Lidington is also due to hold Brexit meetings with figures from the UK’s devolved administrations on Thursday.

The de-facto Deputy Prime Minister was pressed on the DUP’s support when he appeared on ITV’s Peston on Wednesday night. He said Mrs May was committed “heart and soul, not just cerebrally” to the Union and would only put her name to a deal that “treats Northern Ireland on a par as a full part of the United Kingdom”.

Mrs May earlier used her first appearance in the Commons since September’s Salzburg summit to urge MPs to act in the national interest and back a Brexit deal amid warnings that “decisive” progress is needed before a crunch European Council summit in Brussels next week.

Initially framed as the deadline for agreement on the terms of UK withdrawal from the EU, division remaining on issues such as the status of the Irish border, and expectations are growing that the final moment of decision will be put back to a special summit in November.