Scots Tory leader Ruth Davidson and Scottish Secretary David Mundell have warned they could quit their roles if there is a Brexit compromise that threatens to “undermine the integrity” of the UK, according to reports.

The senior Conservative politicians outlined their concerns over the question of the Irish border in a letter to Prime Minister Theresa May last week.

The Sunday Times said the two made it clear they would resign if Northern Ireland faces new controls that separate it from the rest of the UK – because that could fuel the case for Scottish independence.

The newspaper quoted a senior Conservative who said they would consider any differentiated deal that puts a hard border down the Irish Sea to be “a resigning matter”.

Ruth Davidson
Ruth Davidson and David Mundell have reportedly issued a warning over any border down the Irish Sea (Andrew Milligan/PA)

Both have insisted that, more than four years on from the referendum on Scottish independence, keeping the United Kingdom together is their top priority.

The letter states: “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.

“Any deal that delivers a differentiated settlement for Northern Ireland beyond the differences that already exist on an 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.”

It continues: “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.”

Political opponents in the SNP have described the stance as “astonishing” and “bizarre”.

First Minister Nicola Sturgeon tweeted: “‘We’ll resign if you do anything that makes Scotland think there’s a better alternative to a hard Tory Brexit’ say Mundell and Davidson!

“First, what a bizarre position – isn’t it their job to stand up for Scotland? And, second, have they the gumption to resign? I bet they don’t!”

The Scottish Government’s Constitutional Relations Secretary Mike Russell told the BBC’s Sunday Politics Scotland programme: “This is an astonishing development, not that Ruth Davidson holds any position in the UK Government, but to say that you’ll resign if Scotland is not treated as badly as everywhere else strikes me as a complete dereliction of duty.

“They’re elected by people in Scotland and here they want Scotland to suffer the hardest of Brexits. That strikes me as a ludicrous position and one in which we should be saying, have they really come to this?”

Referencing the report on Ms Davidson and Mr Mundell, Labour MP Ian Murray tweeted: “But they also don’t want to continue to participate in the customs union and single market so how do they suggest resolving it? Utterly contemptible.

“They can’t achieve a no hard border in Ireland without participation in the same trading arrangements as the EU.”