BORIS Johnson has left the Commons before the SNP spoke about the effect of the Brexit power grab bill on devolution.

Ministers have admitted the Internal Market Bill, which has its second reading in the Commons today, will breach international law as it overrides key elements of the Withdrawal Agreement agreed between the UK and the EU.

Overriding the Withdrawal Agrrement means a significant shake up of state aid and spending rules, and more power being handed to London over regulations in Scotland, Wales and Northern Ireland.

WATCH: Boris Johnson left speechless as Ed Miliband calls Brexit bill 'hooliganism'

Johnson left after former Labour leader Ed Miliband told him he "doesn’t know his stuff" as the Bill breaks international law.

Miliband highlighted Home Secretary Priti Patel’s call for people to follow new Covid-19 laws, saying: “The Johnson defence means something very specific – there is one rule for the British public and another rule for this Government.

“Pioneered by Cummings, implemented by Johnson, that is the Johnson rule.”

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The Prime Minister was offered the chance to defend his bill but he remained seated, his arms folded.

Johnson left the chamber shortly after the exchange, before Ian Blackford's speech.

Blackford warned Tory MPs not to follow the Prime Minister by breaking the law.

He told the Commons: “This is a test for the House this evening, do not wait for the committee stage. Legally, morally, ethically, the right thing to do is to vote down this Bill tonight and this House must be accountable.

“Do not follow the Prime Minister in acquiescing in breaking the law because if you vote for second reading tonight, that’s exactly what you’re all doing. So this is a test and I understand the challenge that Conservative members face – don’t support the Prime Minister by breaking the law this evening, it is as simple as that.”

He said MPs have the power to “stop the Prime Minister in this act of madness".

“Do not follow the Prime Minister in acquiescing in breaking the law because if you vote for second reading tonight that’s exactly what you’re all doing," he added.

“We have the power individually and collectively to stop the Prime Minister in this act of madness this evening.

“This is a matter of principle. It is about this House saying not to breach our legal obligations and I implore the House to do exactly that.”

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Labour’s amendment to block the Bill at second reading was selected by Speaker Sir Lindsay Hoyle for consideration.

Blackford said the SNP would not support Labour’s amendment because of “some of the other conditions that are attached to it”.