SNP Westminster leader Ian Blackford infuriated the Tories yesterday when he suggested Theresa May had not told the truth over Brexit.

During Prime Minister’s Questions, the Ross, Skye and Lochaber MP said the Tory leader had been “misleading the House, inadvertently or otherwise” over the Irish backstop.

He said the legal advice provided to ministers by the Attorney General, Geoffrey Cox, published in full yesterday afternoon, was at odds with previous statements by the Prime Minister.

“This morning we have seen the detail of the legal advice. We have seen the facts that the Government tried to hide,” he said.

“This government is giving Northern Ireland permanent membership of the single market and the customs union.

“The legal advice is clear, it states: ‘Despite statements in the protocol that is not intended to be permanent, in international law would endure indefinitely.’ Since the Prime Minister returned from Brussels with her deal, the Prime Minister has been misleading the house inadvertently or otherwise.”

But accusing another MP of lying is against Parliament’s rules, and Blackford was immediately cautioned by Speaker John Bercow, who told him to withdraw the words.

However, the SNP’s Westminster leader, said: “I did use the word inadvertently, and I repeat it.

“The Prime Minister returned from Brussels with her deal. The Prime Minister has been misleading the house, perhaps inadvertently.”

Bercow told the SNP leader: “There can be no imputation of dishonour. And the insertion of the word ‘perhaps’ suggests the right honourable gentleman wants to keep his options open.

“The option of imputing dishonour does not exist. That word must now be removed.”

He ordered him to “rephrase, continue and complete, briefly.”

Blackford responded: “I say again, inadvertently,” going on to ask the Prime Minister why she “continues to deny Scotland the rights and opportunities her deal offers to other parts of the United Kingdom.”

The National:

May insisted a “careful analysis” of Cox’s statements and answers in the House would reveal no difference with the legal advice.

She added: “I have myself said on the floor of this house that there is indeed no unilateral right to pull out of the backstop.

“What I have also said is that it’s not the intention of either party that the backstop should (a) be used in the first place, or should (b) if it is used, should be anything other than temporary.”

The legal advice – released only after a humiliating defeat in the Commons on Monday night – saw May’s government became the first in history to be found in contempt of Parliament, runs to 33 paragraphs on six pages, considerably shorter than the 43-page position paper released on Monday.

The key passage, which infuriated the DUP and Brexiteer Tories, was Cox’s suggestion that the backstop for Northern Ireland would effectively mean the rest of the UK would need to be treated as a third country for goods passing from the mainland into the province.

“This means regulatory checks would have to take place between NI and GB, normally at airports or ports, although the EU now accepts that many of these could be conducted away from the border,” it read.

It also makes clear that the UK would never be able to unilaterally walk away from the backstop, which despite assurances that “it is not intended to be permanent”, could mean it would “endure indefinitely until a superseding agreement took its place”.

DUP leader Arlene Foster took to Twitter to say those supporting the withdrawal agreement should read the “clear legal advice from the Attorney General”.

She added: “This is a bad deal for Northern Ireland and the entire United Kingdom. We would be trapped indefinitely.”

Meanwhile, leading Brexiteer Boris Johnson refused to be drawn on whether it was better to leave the EU with May’s deal or remain in Europe under current terms.

He told the BBC: “It’s a pretty finely balanced question but there’s still the chance of saving the position.”

He added: “There’s still a chance of getting a much much better deal and we should get on with doing it as fast as possible.”