Theresa May has dismissed growing concerns about Donald Trump’s mental state, insisting she sees a man making decisions in the “best interests of the United States”.

Asked if she believed the doubts being raised about the President's fitness for office were “serious” – following the publication of an explosive book – the Prime Minister replied: “No.”

“I deal with President Trump. What I see is somebody who is committed to ensuring he is taking decisions in the best interests of the United States,” Mrs May told the BBC’s Andrew Marr Show.

She also confirmed Mr Trump would be visiting Britain, but without revealing when, saying: “He will be coming to this country.”

Mrs May faced the awkward questions after the book, by journalist Michael Wolff, claimed that White House staff reported having to treat Mr Trump like a child.

“What they mean by that, he has a need for immediate gratification. It's all about him. This man does not read, does not listen. He's like a pinball, just shooting off the sides,” the author told NBC News.

A clinical professor has briefed members of the US Congress on the potential risks associated with the President’s behaviour.

Bandy Lee, of the Yale School of Medicine, who has studied how to predict and prevent violence, said she and other psychiatrists felt “the danger has become imminent”.

During the interview, Mr Marr asked: “In the States there are quite serious questions being raised about his mental state. Do you think they are serious?” – prompting Mrs May to reply “no”.

The presenter also asked if the Prime Minister believed Mr Trump was a “child or stable genius” – a reference to the President’s much-ridiculed tweets on Saturday.

The President had attempted to assert his mental fitness, claiming: “Actually, throughout my life, my two greatest assets have been mental stability and being, like, really smart. Crooked Hillary Clinton also played these cards very hard and, as everyone knows, went down in flames.

“I went from VERY successful businessman, to top T.V. Star to President of the United States (on my first try). I think that would qualify as not smart, but genius and a very stable genius at that.”

The 25th amendment of the US constitution provides for the removal of a president deemed unfit, if a majority of the cabinet and the vice-president agree.

Talk that Mr Trump could be removed from power in that way has been sparked by ex-White House strategist Steve Bannon, in Mr Wolff’s book, referring to Vice-President Mike Pence as “our fallback guy”.

Controversially, Mrs May invited the President on a state visit to the UK when she met him at the White House immediately after his inauguration last year.

That visit has now been shelved indefinitely, because of the fear of huge public protests and out of a desire to avoid embarrassing the Royal Family.

However, Mr Trump is expected to undertake a “working visit” as early as next month, to attend the opening of the lavish new US embassy on the banks of the Thames in London.