Theresa May’s cabinet will meet on Tuesday amid widespread disquiet among Conservatives and their allies in the DUP at the plans, which would keep the UK in the EU customs union and boost regulatory checks on goods crossing the Irish Sea between Britain and Northern Ireland.
Downing Street has sought to calm speculation that the compromises will form the basis of a breakthrough on the UK’s Brexit withdrawal agreement ahead of a crucial EU summit that starts on Wednesday.
DUP leader Arlene Foster and her colleagues have reacted angrily to speculation about the contents of the proposed agreement, with Ms Foster warning the Prime Minister not to do a “dodgy deal” that undermines Northern Ireland place in the Union.
Under existing treaties including the Good Friday Agreement, Northern Ireland already has separate regulatory regimes shared with the Republic in a number of areas, including electricity and animal health.
However, the EU has said that under a commitment agreed by London to prevent a ‘hard border’ on the island of Ireland, the north will have to effectively remain within the single market in a number of areas affecting trade.
Checks on goods travelling between the north and Britain would need to be enhanced, affecting 100% of livestock and agricultural products, a significant amount of which comes from Scotland.
“Having fought just four years ago to keep our country together, the integrity of our United Kingdom remains the single most important issue for us in these negotiations,” the letter from Davidson and Mundell states.
“Any deal that delivers a differentiated settlement for Northern Ireland beyond the differences that already exist on all Ireland basis (eg Agriculture), or can be brought under the provisions of the Belfast Agreement, would undermine the integrity of our UK internal market and this United Kingdom.”
The letter adds: “We could not support any deal that creates a border of any kind in the Irish Sea and undermines the Union or leads to Northern Ireland having a different relationship with the EU than the rest of the UK, beyond what currently exists.”
Meanwhile, as many as eight cabinet ministers are said to be considering their position over plans to keep the UK in the customs union as part of a ‘backstop’ to ensure goods continue to flow over the Irish land border whatever the future relationship between London and Brussels, without a firm date for when that arrangement would end.
Asked whether any backstop plan keeping the UK in the customs union would need to have a time limit, Health Secretary Matthew Hancock replied: "I certainly hope so".
"There are different ways you can make sure something is credibly time-limited and that is what I want to see,” he added.
David Davis has called for a cabinet ‘mutiny’, telling his former colleagues to "exert their collective authority" to stop unacceptable compromises on customs and Northern Ireland.
SNP deputy leader Keith Brown said: “It is telling that Davidson and Mundell have not threatened resignation to protect Scotland’s place in the customs union or single market – which is eight times bigger than the UK market alone.
“After all of Davidson’s broken promises during the independence and EU referendums, and the hard-line unionism that is becoming clear for all to see, no-one can rely on a word she says.
“The Tories must stop pandering to the will of hard-line Brexiteers. Remaining in the single market and the customs union is essential for the economy, jobs and living standards - and it's high time the Tories put that before their internal squabbles.”