A Brexit deal handing separate trading rules to Northern Ireland could have “unravelled the entire United Kingdom”, Ruth Davidson warned Theresa May.

The Scottish Conservative leader revealed she “made clear” to the Prime Minister that her block of 13 Scottish Tory MPs would not support any such proposal.

Within hours yesterday, the Brexit Secretary insisted any “regulatory alignment” with the EU would apply across the UK, rather than treat Northern Ireland differently – suggesting Ms Davidson's intervention was crucial.

“A markedly separate deal for Northern Ireland - perhaps with membership of the single market - could have unravelled the entire United Kingdom,” Ms Davidson said.

Writing in The Scotsman, she added: “Indeed, the alacrity with which Nicola Sturgeon spotted a political opportunity on Tuesday only served to demonstrate as much.

“That is why I made clear to the Prime Minister yesterday that neither I nor the 13 Scottish Conservative MPs at Westminster could support such an arrangement.”

The comments come after the DUP blocked any deal “which separates Northern Ireland economically or politically from the rest of the United Kingdom”.

It was clear the DUP believed regulatory alignment on both sides of the Irish border, to avoid customs checks and posts, would only apply in Northern Ireland.

Brexit: the deciders Show all 8 1 /8 Brexit: the deciders Brexit: the deciders European Union's chief Brexit negotiator, Michel Barnier Getty Brexit: the deciders French President Emmanuel Macron Getty Brexit: the deciders German Chancellor Angela Merkel Reuters Brexit: the deciders Commission President Jean-Claude Juncker EPA Brexit: the deciders The European Parliament's chief Brexit negotiator Guy Verhofstadt Getty Brexit: the deciders Britain's Prime Minister Theresa May Getty Images Brexit: the deciders Britain's Chancellor of the Exchequer, Philip Hammond PA Brexit: the deciders After the first and second appointed Brexit secretaries resigned (David Davis and Dominic Raab respectively), Stephen Barclay is currently heading up the position PA

The veto plunged the Prime Minister into a fresh crisis, with a new EU deadline to present an acceptable plan to avoid a hard Irish land border – with customs checks - looming at the end of the week.

Ms Sturgeon, the SNP First Minister, had leapt on the original suggestion to claim it left “no good practical reason” for Scotland not to be allowed the same deal softer Brexit deal.

In the article, Ms Davidson argued that keeping the internal market throughout the UK on a level playing field is crucial to Scotland.

“My view is therefore that, whatever happens with Brexit, we should do nothing that damages the integrity of the one union that really does keep Scotland in business,” the Tory leader wrote.