IAN Blackford was forced to back down in the Commons after branding Theresa May a “liar” during heated exchanges on Brexit.

The SNP leader at Westminster was responding to the Prime Minister as she updated MPs on the revived talks with Brussels in her bid to get the backstop changed.

He initially claimed Mrs May was living in a “parallel universe” and when she laughed, the Highland MP declared: “She sits there laughing. Sometimes you should be honest with yourself, never mind being honest with the people of the United Kingdom.”

As the Tory benches expressed their disapproval at his tone, Mr Blackford continued, accusing the PM of living in a “Brexit fantasy”.

“We are 45 days from Scotland being dragged out of the European Union against our will, 45 days from economic catastrophe,” he declared.

Branding her deal a “fraud,” and calling on her to extend the Article 50 process, the Highland MP questioned whether the Government had undertaken an economic impact analysis on her deal.

When Mrs May rose to insist such an analysis had been published, Mr Blackford shouted out: “Not true.”

After John Bercow, the Commons Speaker, intervened to insist the PM must be heard, she suggested by his interruption the SNP leader might have “inadvertently misled the House”.

At this point, Mr Blackford shouted out: “Liar.”

As Tory MPs reacted angrily and pointed their fingers at the Nationalist MP, Mr Bercow rose to “ask the right honourable gentleman to withdraw that word. He cannot accuse another Member in this House of dishonesty. Withdraw”.

“In courtesy to yourself, I withdraw,” said the SNP leader to cries of dissatisfaction from the Conservative benches.

But the Speaker intervened again to say there were plenty of precedents for such a response, including doing it once himself, adding: “That is enough.”

Mrs May rose to insist again an economic analysis of her deal had been published and added that Mr Blackford’s assertion that Scotland should be independent of the UK might raise cheers on the SNP benches but it would, she insisted, “not raise cheers from those people in Scotland whose economic future depends on[Scotland] being a member of the UK”.

Later, Tory backbencher Stephen Kerr condemned Mr Blackford for his “boorish behaviour” and “crass nonsense”.

The Stirling MP added: “People are sick and tired of the SNP and their contrived approach to politics, full of grievance and pettiness. Scotland deserves far better.”