In a shot across the prime minister’s bow, DUP MP Sammy Wilson joined forces with Tory Brexiteer Steve Baker, deputy chairman of the 80-member European Research Group (ERG) of Conservative backbenchers, to warn that they will vote against any withdrawal agreement which they believed threatens the Union.

As pressure intensifies on the PM, the two MPs – writing in the Sunday Telegraph – said: “We share the prime minister’s ambition for an EU free trade agreement, but not at any price, and certainly not at the price of our Union.

“If the government makes the historic mistake of prioritising placating the EU over establishing an independent and whole UK, then, regrettably, we must vote against the deal.”

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Prime Minister Theresa May is facing mounting pressure to ditch her Brexit plan or face defeat in Parliament

The combined intervention comes as former foreign secretary Boris Johnson claimed Mrs May is on the brink of “total surrender” to the EU over Brexit, as he urged the Cabinet to mutiny against her withdrawal agenda.

Mrs May is under fire from both wings of the Tory party after the shock resignation from the government of Mr Johnson’s pro-European brother Jo, who also delivered a withering attack on the PM’s stance.

Speaking to the News Letter, Mr Wilson said he was “baffled” by the PM’s current strategy and claimed her plan would never secure passage through Parliament.

The DUP’s main problem with Mrs May’s Brexit blueprint centres on the possibility that NI could be forced to follow EU single market rules post-Brexit, essentially leading to a regulatory border in the Irish Sea.

But Mr Wilson said his party was heartened by the growing swell of opposition to the plan, with many in Mrs May’s own party uneasy at the prospect of the UK staying in a temporary customs arrangement with the EU.

He pointed out that Scottish Tories could vote down the Brexit deal if it included extending the Common Fisheries Policy.

“The PM must realise that she has no hope of getting this through the Commons, and yet she ploughs on,” he added.

“I am at a loss to understand what her tactic is. Maybe she is hoping that a mass of Labour MPs will rescue her. I think she might get away with that if she had only lost the support of a dozen MPs, but she has lost the support of up to 60 or 70 of them.”

Mr Wilson said his party was seeking to galvanise opposition to the plan, adding that the party had made “good progress” on that front.

Hopes of getting the Cabinet to sign off on Brexit deal proposals this week appear to be rapidly receding, as it was reported the EU had rejected London’s plans for an independent arbitration clause that could allow the UK to quit a backstop deal on the Irish border.

The backstop arrangement – which has been the main sticking point in the Brexit negotiations – is designed to be put in place to prevent a hard border on the island of Ireland in the event of a no-deal Brexit.

Talks between senior British and EU officials in Brussels continued until 2.45am on Monday, Downing Street has said.