THERESA May was challenged to sack Boris Johnson after he described her Brexit plan for a customs partnership as “crazy” in a newspaper interview.

The call was made by Ian Blackford, the SNP’s Westminster leader, who pressed her during Prime Minister’s Questions yesterday, asking if “she had enough backbone” to send her Foreign Secretary to the backbenches for undermining her and for a vain attempt to plead with Donald Trump via a television show not to pull out of the Iran nuclear deal.

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May sidestepped the comments made against her policy by Johnson, saying it was “absolutely right” that representations were made to Trump on a variety of levels regarding the Iran deal, which the US President tore up on Tuesday.

The Cabinet split over the Prime Minister’s customs partnership plan, which would see the UK collect tariffs on behalf of the EU, was also seized on by Jeremy Corbyn in his questions to May, with the Labour leader picking up on the views expressed by Johnson that the plan was “crazy”.

But May tried to turn the debate away from Johnson’s comments and instead attacked Corbyn’s record as a backbencher, claiming he now wanted the UK to stay in the customs union despite spending “an entire career” opposing such an arrangement.

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She also accused Labour of “letting Britain down once again”, telling MPs the Opposition’s Brexit strategy meant it would fail to meet its pledge to strike trade deals.

Brexit dominated the leaders’ exchanges at Prime Minister’s Questions, which saw deputy speaker Sir Lindsay Hoyle stand in due to Speaker John Bercow travelling to Glasgow to attend the funeral of his predecessor, Michael Martin.

In his opening question, Corbyn asked: “Does the Prime Minister agree with her Foreign Secretary that the plan for a customs partnership, set out in her Lancaster House speech, is, in fact, crazy?”

May replied: “We are leaving the European Union, we are leaving the customs union, but of course for our future trade arrangements, trade relationship with the European Union, we will need to agree customs arrangements which will ensure that we leave the customs union, that we can have an independent free trade policy, that we can maintain no hard border between Northern Ireland and Ireland, and that we have as frictionless trade with the European Union as possible.

“I’ll tell [Corbyn] what’s crazy – what’s crazy is a Leader of the Opposition who for years opposed TTIP [the Transatlantic Trade and Investment Partnership] and now has a policy that would mean Labour signing up to TTIP with no say in it whatsoever.”

Corbyn said Greg Clark appeared to back the “crazy customs partnership” proposal but had made clear he did not back a technological alternative, prompting May to insist the Business Secretary had said the UK would be leaving the customs union.

In Glasgow yesterday around 5000 people attended the European Commission-organised Europe Day to highlight the benefits of EU membership.

Speaking at the event Fiona Hyslop, Scotland’s External Affairs Secretary, said ministers would be stepping up engagement with EU countries further, and that it was more important than ever for Scotland’s voice to be heard.

“The strong desire by our European partners to engage has never been greater,” she said.

“People in Scotland voted overwhelmingly to remain in the EU and there were majorities for remain in every local authority area.

“We are determined to make Scotland’s voice heard as we enter a crucial stage in the Brexit negotiations and will be stepping up our engagement plans.

“Whatever the outcome, it is essential that we continue our collaboration, friendship and partnership with EU nations.”