The Shadow Foreign Secretary Emily Thornberry has said her party would take a much firmer line with President Donald Trump were it in government, and she would not be afraid to say where she disagreed with him.

She accused the President of “completely inappropriate language” over the North Korea crisis, warning that the world was only a miscalculation away from “terrible consequences”.

Ms Thornberry suggested members of Mr Trump’s administration believed the President had a deliberate policy to portray himself as “crazy”.

“He is playing games, he is using intemperate, completely inappropriate language in very dangerous times because the difficulty will be how will North Korea interpret that?” she said, speaking to Sky News. “And there could be a miscalculation, a misunderstanding that could lead us into terrible circumstances. This is not the way for the leader of the free world to behave.”

She suggested he was performing a “good cop, bad cop” routine with secretary of state Rex Tillerson.

“We shouldn’t be proceeding with a President who is putting himself forward as – and I think some people in the administration are saying it’s a deliberate policy that he’s putting himself forward as – crazy. I mean, what?”

Insisting Labour would take a strong line with the President, she said that as foreign secretary she would “make it completely clear the way in which I disagree with him, I disagree with him on a very large number of things. I would say how appalling it is that he is trying to undermine the Iranian nuclear deal, for example, or Paris climate change.”

She added: “The United States is not just Donald Trump. There are many good people and good people of good will in the United States and we make friends with them and we work with them but we make it clear to Donald Trump, some of his utterances are completely unacceptable and totally different to our values and what a shame it is that Theresa May feels it necessary to hold his hand.”

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She also said Theresa May should resign as she “doesn’t have enough authority” to lead her party and country through Brexit, adding: ”It doesn’t really matter who their leader is, they don’t know which way they are going, they don’t know what it is that they want to achieve.

“There’s definitely a chunk of them that want to have no deal at all, which would be definitely contrary to British interests.”

Ms Thornberry said Ms May should resign and “let the grownups have a go” at negotiating Brexit.