Shadow Scottish secretary Lesley Laird will accuse David Mundell of abandoning his “red lines” over the status of Northern Ireland under the Prime Minister’s deal, saying it “risks undermining the integrity of the UK”.

Her comments follow publication of a poll showing that most Scottish voters would prefer independence to either a no-deal Brexit or Mrs May’s deal, although most people said they would back the Union when asked how they would vote in a fresh independence referendum.

Yesterday First Minister Nicola Sturgeon said voters would have the chance to opt for independence to avoid a post-Brexit future that she likened to “Narnia for idiots”.

It came after newspaper reports that Boris Johnson’s friends had compared him to Aslan from CS Lewis’s children’s book The Lion, the Witch and the Wardrobe, claiming he was ready to topple the Ice Queen – Mrs May.

“It’s hard to know whether to laugh or cry,” Ms Sturgeon wrote on Twitter. “These idiots are actually revelling in the idea that they’re characters in a fantasy world.

“Scotland, we don’t have to stay in Narnia with them – we can opt to stay in the real world with independence.”

Several Scottish MPs are set to speak in the Commons on the penultimate day of debate on Mrs May’s deal, including Scottish Tories who have yet to declare how they will vote tomorrow.

Ms Laird said yesterday: “David Mundell is supposed to be Scotland’s man in the Cabinet, but that could not be further from the truth.

“His ‘red lines’ on the integrity of the UK and the fishing industry must have been written in invisible ink.

“After 20 months of negotiations with the EU, the Prime Minister has come back with a half-baked deal that puts jobs and our economy at risk and risks undermining the integrity of the UK.”

The Scottish Secretary defended his decision to stay in the Cabinet despite co-signing a letter to the Prime Minister with Scottish Conservative leader Ruth Davidson warning against 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”.

Mr Mundell said it had “always been a condition of the withdrawal agreement” that it would contain a backstop arrangement for Northern Ireland, to prevent the return to a hard border there.

The Scottish Secretary accused the SNP of exploiting Brexit by voting against the Prime Minister’s deal, and said he “had to make a judgment on what is in the backstop, which is a temporary measure which may never come into force, and what risk that causes to the integrity of the United Kingdom”.

He added that the future of the Union “is most likely to be threatened by the crash, by the uncertainty, by the division, by the chaos which provides the backdrop Nationalists want to move forward with their independence referendum here”.

Mr Mundell said it was “simply not possible” for the UK to renegotiate the Brexit deal and remove the backstop, as has been claimed by the former Foreign Secretary and others. He said: “Mr Johnson knows that, he was in the Cabinet till relatively recently. I don’t understand how he has come to that conclusion.”

Asked if he would remain in the Cabinet if Mr Johnson became prime minister, Mr Mundell said: “I don’t see myself being able to serve in that way.”

Meanwhile, a Panelbase survey of 1,028 voters found 59 per cent of respondents agreed that Scottish independence would be better than a no-deal Brexit, with 41 per cent disagreeing. When asked if independence would benefit the country more than staying in the UK after a negotiated Brexit deal, 53 per cent agreed and 47 per cent disagreed.

The poll found 53 per cent of voters still intend to back staying in the UK in a new independence referendum. However, support for independence is at the highest level recorded by Panelbase in two years, once the don’t knows are removed, at 47 per cent.

SNP depute leader Keith Brown said: “The simple fact is that Scotland voted overwhelmingly to remain in the EU and is having its wishes ignored by Westminster.

“There is nothing that highlights Scotland’s democratic deficit as starkly as Brexit – and the untold damage it will inflict upon jobs, living standards and our NHS. No wonder support for independence is at historically high levels.

Pamela Nash, chief executive of campaign group Scotland in Union, said: “With uncertainty over Brexit, it’s clear that Nationalists are trying to capitalise on that in the desperate hope of boosting support for their campaign to leave the UK. Whatever your views on Brexit, independence is not the answer.”

Just over half of Scots (51 per cent) would back a fresh general election if the Prime Minister fails to get her Brexit deal through parliament, and 54 per cent believe she should resign if defeated, according to Panelbase.

The SNP would made modest gains in a general election, with the poll putting their support at 37 per cent, Labour and the Conservatives tied on 26 per cent, the Liberal Democrats on 6 per cent and Ukip and the Greens on 2 per cent. But it found that Ms Sturgeon risks losing her pro-independence majority in the next Holyrood election, due in 2021.

Labour warn UK at risk as Scots pick independence over Brexit