RUTH Davidson’s warning to Theresa May on the Irish border issue has forced the UK Government to reaffirm its commitment to the “constitutional integrity” of the United Kingdom.

After the Scottish Conservative leader spoke by telephone to the Prime Minister and issued a statement making clear any “regulatory alignment” with the EU on trade post Brexit should apply not just to Northern Ireland but to the whole of the UK, David Mundell, the Scottish Secretary, spoke in Cabinet, telling colleagues that nothing should be done to undermine the Union.

Later, David Davis declared: “The integrity of the United Kingdom comes first,” and suggested to MPs that any regulatory alignment in certain areas would apply across the whole of the UK.

During Commons exchanges, the Brexit Secretary said: “Every approach we take will treat the whole United Kingdom as a single constitutional entity and a single economic entity.”

He made clear that any suggestion the UK Government in leaving the EU might leave “one part of the United Kingdom behind, still inside the single market and customs emphatically not something that the UK Government are considering.

“So when…the First Minister of Scotland says it is a reason to start banging the tattered drum of independence or the Mayor of London says it justifies a hard border around the M25, I say they are making a foolish mistake. No UK Government would allow such a thing, let alone a Conservative and Unionist one.”

Tory sources made clear Ms Davidson had been deeply concerned at the UK Government’s approach in Brussels. One insider said the Scottish Tory leader, whose central political purpose has been and continues to be the maintenance of the Union against the SNP threat, did not want to see “the thread of the Union unravelled by Brexit”.

One MP told The Herald: “Ruth made it clear to the PM that the Conservatives are first and foremost a Unionist Party,” while another noted how Mr Davis, sparked by the Scottish party leader’s intervention, had given a “clear restatement that the Union is the first and ultimate priority”.

Last night, the Scottish Conservative group of MPs released a statement, saying it had met and “unanimously agreed that we fully support the comments from Ruth Davidson MSP that the terms of any Brexit deal with the EU should be UK-wide”.

Earlier in Edinburgh as MSPs debated the Brexit developments, the First Minister noted: "The Scottish Tories seem more interested in appeasing the DUP than in standing up for Scotland's best interests. Scotland's interests demand continued single market membership."

On Monday the talks in Brussels to get agreement to push the Brexit negotiations onto trade and transition dramatically broke down after the Democratic Unionists made clear they could not agree to any deal that treated Northern Ireland separately from the UK.

Yesterday evening, Arlene Foster, the DUP leader, said she had tried for weeks to get sight of the draft UK-EU deal and when she saw it, it came as a “big shock”.

She explained: “When we looked at the wording[on regulatory alignment] and had seen the import of all that, we knew we couldn’t sign up to anything that was in that text, that would allow a border to develop in the Irish Sea.”

A mooted telephone call between Mrs May and Ms Foster due to take place yesterday had, by early evening, still not taken place.

Earlier in the Commons after Labour successfully lodged an Urgent Question, Mr Davis claimed the UK was now "close" to concluding the first phase of Brexit negotiations, dealing with the Irish border, citizens' rights and the UK's financial settlement.

He explained how “regulatory alignment" did not mean full harmonisation with EU regulations.

"It's sometimes having mutually recognised rules, mutually recognised inspection, all of that sort of thing as well and that's what we are aiming at. There are areas where we want the same outcome but by different regulatory methods.”

But Sir Keir Starmer for Labour urged Mrs May to "rethink her reckless red lines" and put the option back on the table of the UK remaining within the European single market and customs union.

He argued that the events of the last 48 hours had shown the "DUP tail is wagging the Conservative dog".

Peter Grant for the SNP, describing the Government’s Brexit strategy as a “shambles,” mocked Mrs May, suggesting she was being interviewed for the job of Scotland’s football manager, “where her fantastic ability to snatch defeat from the jaws of victory could be put to very good use”.

Mrs May is due to visit Brussels again later this week to try to finalise the first phase of the divorce deal, which would allow the EU27 leaders next week to give the green light to the second phase on trade and transition.

But Downing Street acknowledged negotiations could go right to the wire at the European Council on December 14/15.