DUP and Conservative Brexiteer MPs came together yesterday to step up pressure on the Prime Minister over her faltering Brexit strategy.
In a stark warning to the PM, the DUP's Brexit spokesman, Sammy Wilson, and Tory Brexiteer Steve Baker, deputy chairman of the 80-member European Research Group (ERG) of Conservative backbenchers, said they would vote against any agreement which they believed threatened the Union, and could put a trade border down the Irish Sea.
Writing in The Sunday Telegraph, the two MPs 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."
Hope of getting the Cabinet to sign off on Brexit deal proposals this week appeared 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 Northern Ireland border.
The DUP/ERG move comes as the Prime Minister battles to keep her Brexit agenda on track in the face of growing Tory tensions and reports of opposition from Brussels to a key part of her withdrawal proposals.
The hardening line against the Prime Minster's plan comes as the representatives of the four anti-Brexit Northern Ireland parties fly into London for a meeting with Mrs May.
Speaking ahead of the No.10 meeting, Alliance MLA Dr Stephen Farry said: "The government's Brexit negotiating approach has always been hamstrung due to mutually incompatible red lines.
"Over the next few weeks, in order to achieve a Withdrawal Agreement and the Transition Deal, the UK government has to honour its existing commitments to be open-ended and all weather backstop.
"Alliance welcomes some aspects of the backstop extending UK wide, provided the protection for Northern Ireland is maintained. UK-wide, we have an almost farcical situation in which a majority of MPs seem to want to at least keep all of the UK in a Customs Union and within the Single Market, and there is significant evidence to suggest that the people of UK would vote differently in another referendum. But democracy is being frustrated as the government and hard Brexiteers are taking the UK to the cliff edge."