IAN Blackford accused Theresa May of being a “liar” during a heated exchange on Brexit in the Commons.

The Prime Minister had come to Parliament to ask MPs to "hold their nerve" and let her have more time for negotiations with the EU.

The SNP leader was palpably angry with May, questioning her honesty four times in his short statement.

It started partly because she claimed that she had wanted Brexit sorted before December.

May was, he said, living in a “parallel universe”.

“We’ve just heard from the Prime Minister that she wanted this concluded in December, talk about rewriting history," he added.

"It was the Prime Minister that denied us the right to have the meaningful vote. And to try and rewrite history, and she sits there laughing.

"You know, sometimes you should be honest with yourself, never mind being honest with the people of the United Kingdom."

Blackford then asked the Prime Minister if the UK Government had an economic assessment of the impact of her deal on the UK economy: “You’re asking this House to vote on your deal and you can’t even be honest on the economic impact.

WATCH NOW: Ian Blackford calls Theresa May a ‘liar’ during heated Commons exchange

Responding to the SNP chief, May said the Government had published economic assesments of the proposals that they had put forward.

A furious Blackford shouted “that’s not true”.

May then hit back, saying the MP for Ross, Skye and Lochaber, had “inadvertently misled the House” prompting Blackford to call her a “liar”.

Parliament’s strict rules forbid MPs from questioning the honesty of their colleagues. A politician suggesting another is a untruthful can find themselves kicked out of the chamber.

“If that word was used without equivocation or qualification that word must be withdrawn, at once. At once,” Bercow said.

He added: “If a member on the front bench used, that word – I’m sorry, I’m not debating, I’m not arguing, I’m not negotiating, that word must be withdrawn. At once.

“That word was used by the leader of the SNP, if so and I want the debate to continue and it will, if so, I simply ask the right honourable gentleman to withdraw that word. He cannot accuse another member of this house of dishonesty.”

Blackford then offered a qualified withdrawal of his accusation, saying it was only out of “courtesy” to the Speaker.

That infuriated Tory MPs who felt it was an insincere apology, but Bercow allowed it.

In her statement, May had asked for "some time" to hold talks with the EU, promising to return to Parliament on February 26, if no deal has been secured before that time, to report back on progress and trigger a further vote by MPs the following day.

"We now all need to hold our nerve to get the changes this House requires and deliver Brexit on time," May told the Commons.

"By getting the changes we need to the backstop; by protecting and enhancing workers' rights and environmental protections; and by enhancing the role of Parliament in the next phase of negotiations I believe we can reach a deal that this House can support.

"We can deliver for the people and the communities that voted for change two-and-a-half years ago – and whose voices for too long have not been heard.

WATCH NOW: Ian Blackford accuses Theresa May of 'rewriting history'

"We can honour the result of the referendum.

"And we can set this country on course for the bright future that every part of this United Kingdom deserves.

"That is this government's mission. We shall not stint in our efforts to fulfil it."

Labour leader Jeremy Corbyn accused the PM of "running down the clock" in the hope that MPs will be "blackmailed" by the fear of a no-deal Brexit into supporting "a deeply flawed deal".

"This is an irresponsible act," said Corbyn. "She is playing for time and playing with people's jobs, our economic security and the future of our industry."