David Mundell said he'd resign over his Brexit red lines ...

The Tory minister is coming under pressure to keep promises to quit the Government if the Prime Minister’s agreement with Europe led “to Northern Ireland having a different relationship with the EU than the rest of the UK”.

Legal advice given to Cabinet by Attorney General Geoffrey Cox, and released by the Government in full on Tuesday, made clear that if the backstop was to come into force then Northern Ireland would effectively remain in the EU single market for goods and the rest of the UK would be “essentially treated as an third country by NI [Northern Ireland] for goods passing from GB into NI”.

But yesterday Mundell insisted the deal was “carefully tailored” and didn’t breach his red lines.

Following a speech to business leaders, Mundell admitted he was not “totally comfortable” with the details around Northern Ireland.

He said: “I have looked at the detail of the proposals for Northern Ireland and I have been quite clear I am not totally comfortable with those arrangements.

“If you weigh up what’s the greatest threat to the integrity of the UK, my judgement is that’s having no-deal, chaotic Brexit from the EU – I regard that as the most unsatisfactory outcome in terms of standing up for the integrity of the UK.”

When asked why May refused to rule out the damaging no-deal Brexit he claimed to want to avoid, Mundell said the draft deal was the only solution.

“She’s making the case for her deal because her deal is the best way to avoid that threat,” he said.

“Even Mrs May’s most passionate critic couldn’t say she has not done that as vigorously and with as much effort as could possibly have been done because ... She believes that voting for that deal is the way to resolve these issues.”

Earlier, speaking to the BBC’s Good Morning Scotland, Mundell defended the Prime Minister against accusations that she had lied to Parliament. He insisted the Attorney General’s legal advice on the Northern Ireland backstop was not “broadly different” from a previously published summary and that people had known the basis on which the Government was proceeding.

On Wednesday, during Prime Minister’s Questions, the SNP’s Ian Blackford accused May of “misleading” MPs. He then went further, telling The Herald that the Tory leader had “lied”.

Mundell said: “I think the Prime Minister has been very clear and very open throughout the process and has not hidden the things within the deal that people might or might not like.

“She has not hidden that this is a hard decision, it’s not a straightforward decision, but I also think she has been very clear about what the reality is if people don’t support this deal.

“I don’t think anybody has been misled – if there is any attempt at misleading it is from the SNP who are somehow projecting they want some other, better alternative yet they’re quite prepared to impose a no deal on Scotland.”

The SNP MSP Tom Arthur said Mundell was becoming “increasingly delusional” and compared him to Saddam Hussein’s PR man, Comical Ali.

“The UK Government’s legal advice confirms what we already knew – that Theresa May’s deal doesn’t just breach Mundell’s red line, it demolishes it.

“If David Mundell had a shred of credibility he would have resigned weeks ago, but instead is hanging on and claiming black is white.“

Mundell was, he added, “fast becoming the Comical Ali of Brexit”.