The author of a new book detailing the first days of the Trump administration says in a new interview that there are discussions even in the White House about whether the president has the mental stamina required for the job.

Michael Wolff, author of "Fire and Fury: Inside the Trump White House," told BBC's Radio 4 that during his time reporting within the White House, President Trump Donald John TrumpHR McMaster says president's policy to withdraw troops from Afghanistan is 'unwise' Cast of 'Parks and Rec' reunite for virtual town hall to address Wisconsin voters Biden says Trump should step down over coronavirus response MORE's aides questioned the president's mental fortitude in the face of increasingly erratic behavior.

"The truth is, over this period that I witnessed, this seven or eight months, they all came to the conclusion — gradually at first, then faster and faster — that something was unbelievably amiss here," Wolff said.

"That this was more peculiar than they ever imagined it could be. And at the end, they had to look at Donald Trump and say, 'No, this is a man who can't function in his job as president.' He may have been elected president, but that does not turn him into president," he added.

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He also responded to a question from the show's host on whether he would describe Trump as "childlike."

"I don't describe him as childlike, every person in the White House ... Literally, that is the common description among every one of his senior people," Wolff said. "That the president is somewhat like a child."

"In what way?" the host responded.

"Sometimes it's an 11-year-old, sometimes it's a 6-year-old, sometimes it's a 2-year-old," Wolff continued. "And it all depends on his need for immediate gratification. 'I want what I want when I want it.' "

The White House has pushed back against Wolff's book, with Trump himself denying that the author had West Wing access or interviewed Trump.

“Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book. He used Sloppy Steve Bannon, who cried when he got fired and begged for his job. Now Sloppy Steve has been dumped like a dog by almost everyone. Too bad!” the president tweeted Saturday.

Wolff has said that the White House's response to his reporting simply proves him correct.

“Not only is he helping me sell books, he is helping me to prove the point of the book,” Wolff said on the Today show Friday. “This is extraordinary, that the president of the United States would try to stop the publication of a book.”