Theresa May has been branded a “liar” by the SNP leadership after a bitter row broke out over the UK Government’s legal advice, which UK ministers have been forced to publish.

And as the Prime Minister held private talks with Tory rebels on the possible creation of a “parliamentary lock” - a Commons veto on the Irish backstop - Ian Blackford, the Nationalist leader at Westminster, made clear a People’s Vote was now his party’s preferred choice, saying it was the “best option,” should, as is expected, Mrs May’s Brexit Plan be rejected by MPs in next Tuesday’s crunch vote.

In another day of high drama at Westminster, Mr Blackford clashed with John Bercow, the Commons Speaker, after he suggested the PM had deliberately misled MPs; an allegation that breaches parliamentary protocol.

During PMQs, the Highland MP claimed Mrs May had misled the House “inadvertently or otherwise”.

As Tory MPs shouted “withdraw,” the Speaker intervened to rebuke the SNP leader, stressing there could be “no ambiguity" in comments to suggest the PM had purposefully misled the House.

This led Mr Blackford to alter his wording to say she had done it "perhaps inadvertently".

However, this led to even louder jeers from the Tory benches and Mr Bercow asked him again to "rephrase" his words. The MP for Ross, Skye and Lochaber continued unchallenged after removing the word “perhaps” from his subsequent remarks.

HeraldScotland:

In her response, the PM stressed how there was no difference between what Geoffrey Cox, the Attorney General, had said in a statement to the Commons on Monday and the published legal advice.

She added: “I have said on the Floor of the House that there is no unilateral right to pull out of the backstop. I have also said that it is not the intention of either party for the backstop to be used in the first place or, if it is used, to be anything other than temporary.”

Later, outwith the chamber, Mr Blackford declared: “It’s absolutely conclusive that the PM has misled the House relating to the permanency of the backstop.”

Noting how Mrs May had told MPs that the backstop would be “only temporary,” the party leader was asked if he was, therefore, saying the PM was lying.

He replied: “Let us call a spade a spade. On the basis…she had the legal advice when she spoke in the House of Commons, then she’s lied, yes.”

The published paper from Mr Cox - in bold type - states: “Despite statements in the Protocol that it[the backstop] is not intended to be permanent and the clear intention of the parties that it should be replaced by alternative, permanent arrangements, in international law the Protocol would endure indefinitely until a superseding agreement took its place…”

The six-page document also said that under the proposed arrangements, "for regulatory purposes, GB is essentially treated as a third country by NI for goods passing from GB into NI".

Nigel Dodds, the Democratic Unionist Party leader at Westminster, described the legal advice as "devastating" and said it made clear the proposed backstop was "unacceptable" and must be defeated.

Sir Keir Starmer for Labour said the legal advice had confirmed "the central weaknesses in the Government's deal".

As the PM continued her Commons meetings with Tory MPs, saying she was “listening to the concerns” of colleagues about “the way forward,” it emerged No 10 was discussing a plan to give MPs a vote on whether the UK should enter the backstop at all or if the Brexit Plan should be scrapped altogether.

But ardent Brexiteer Steve Baker dismissed the Downing St strategy, saying: “It is silly and few are falling for it.”

Meanwhile, it was suggested EU chiefs are preparing for a May defeat by putting the issue of extending Article 50 on the agenda for the December European Council, which takes place two days after next Tuesday’s Commons vote.

The continuing row at Westminster came as Holyrood formally voted against both Mrs May's Brexit deal and the prospect of quitting the EU with no deal in place.

MSPs voted by 92 to 29 for a motion which said both these options "would be damaging for Scotland and the nations and regions of the UK as a whole".

As a result, they called for the Brexit deal to be rejected when it is voted on at Westminster on December 11 and for "a better alternative be taken forward".

Today, David Mundell will open up a new front in the Brexit row when he calls on Nicola Sturgeon to “open her ears” to Scottish business and back the PM’s withdrawal plan.

In a speech in London, the Scottish Secretary will say: “If she genuinely wants to represent Scotland’s views she has to stop lecturing and start listening.

“If she listened, she’d know we have a workable solution. If she listened, she’d understand how little appetite there is for simply prolonging the uncertainty. She also claims we must avoid no-deal. If she listened, she’d realise the only sure way to achieve that is to support the deal.”

In response, the SNP MSP Tom Arthur declared: “David Mundell is so delusional he is fast becoming the Comical Ali of Brexit. His claims that this is a choice between the PM’s Bad Deal and no-deal have already been completely demolished by Theresa May’s humiliating Commons defeats and by the opinion from the European Court of Justice.”