Brexit: Ruth Davidson hints she will resign if deal gives Northern Ireland special trading terms with EU

'We could not support any deal that leads to Northern Ireland having a different relationship with the EU than the rest of the UK'

Ruth Davidson has deepened Theresa May’s troubles by hinting she will resign if the Brexit deal gives Northern Ireland special trading terms with the EU.

The Scottish Tory leader said she “could not support” such an agreement, in a joint letter also signed by David Mundell, the Scottish secretary.

“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 pair wrote.

The letter, obtained by The Scotsman, provides evidence to back up reports that both Ms Davidson and Ms Mundell are prepared to resign if the prime minister refuses to back down.

It was revealed as Dominic Raab, the Brexit secretary, headed to Brussels to try to break the deadlock in the talks, with just three days until a crunch summit of EU leaders.

Under Ms May’s plan – also opposed by the Democratic Unionist Party (DUP) – Northern Ireland would effectively stay in the EU single market as part of the “backstop” to avoid a hard Irish border.

The Tories in Scotland are under pressure because the SNP has already questioned why such an arrangement is not being offered to Scotland as well.

Ms Davidson and Mr Mundell will fear it will bolster the case for independence north of the border, if the backstop creates distinct single market terms for Northern Ireland only.

The letter says: “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.”

And it 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.”

The resignation of the popular Ms Davidson – who has led a Tory revival in Scotland and successfully softened the party’s image – would be a huge blow to the prime minister.

The threat emphasises how Ms May now faces cabinet opposition to her backstop plan for two separate reasons, ahead of a showdown on Tuesday.

The rest of the nine ministers challenging it fear the entire UK will be locked into the EU’s customs union for many years to come, unless the backstop has a legal end date.

The Independent revealed on Friday that Andrea Leadsom, the Commons leader, is prepared to resign if Ms May compromises further.

Esther McVey, the work and pensions secretary, Penny Mordaunt, the international development secretary, and Liam Fox, the international trade secretary, are also on “resignation watch”.

On Friday Dominic Raab, the Brexit secretary, insisted the “backstop” – the key hurdle to a withdrawal agreement – must have an end date or it would be rejected by parliament.

But Mr Mundell’s and Ms Davidson’s opposition is to the linked proposal for Northern Ireland to retain EU regulations on goods and agriculture, with checks on flows across the Irish Sea.