British Prime Minister Theresa May has dismissed concerns about Donald Trump's mental fitness, saying the U.S. president acts in what he sees as the best interests of his country.

A new book by journalist Michael Wolff quotes prominent Trump advisers as questioning the president's competence.

Asked in an interview whether she thought concerns about Trump's mental state were serious, May said: "No."

She said that "when I deal with President Trump what I see is somebody who is committed to ensuring that he is taking decisions in the best interests of the United States."

In the BBC interview broadcast Sunday, May reaffirmed that Trump would visit Britain. She did not give a date, or say whether it would be a full state visit or a lower-key working trip.