LONDON/WASHINGTON (REUTERS/BLOOMBERG) - The author of a book that is highly critical of Donald Trump's first year as US president said his revelations were likely to bring an end to Trump's time in the White House.

Michael Wolff told BBC radio that his conclusion in "Fire and Fury: Inside the Trump White House" that Trump is not fit to do the job was becoming a widespread view.

"I think one of the interesting effects of the book so far is a very clear emperor-has-no-clothes effect," Wolff said in an interview broadcast on Saturday (Jan 6). The book depicts a chaotic White House, a president who was ill-prepared to win the office in 2016, and Trump aides who scorned his abilities.

"The story that I have told seems to present this presidency in such a way that it says he can't do his job," Wolff said."Suddenly everywhere people are going 'oh my God, it's true, he has no clothes'.

"That's the background to the perception and the understanding that will finally end ... this presidency."

'Boring and untruthful book'

Trump has dismissed the book as "phony" and "full of lies, misrepresentations and sources that don't exist". He took to Twitter late on Friday to renew his attacks on Wolff, and on his former top aide Steve Bannon who was quoted in the book.

"Michael Wolff is a total loser who made up stories in order to sell this really boring and untruthful book," Trump said. "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!"

Bannon, formerly Trump's chief strategist, is chairman of the so-called alt-right Breitbart News website In his interview with the BBC, Wolff was asked if he believed that Bannon felt Trump was unfit to serve as president and would try to bring him down.

"Yes," Wolff replied.

He also hit back at claims that the book was untruthful. "This is what's called reporting. This is how you do it." he said. "You ask people, you get as close as you can to the event, you interview the people who were privy to the event, you interview other people who were privy to the event, you come to know the circumstance as well as anybody and then you report it."

Wolff had told American network NBC on Friday that "100 per cent of the people around" the president question his intelligence and fitness for office.

"They all say he is like a child," Wolff said on NBC. "What they mean by that is 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," Wolff added. "They say he's a moron, an idiot."

White House press secretary Sarah Huckabee Sanders said on Fox News on Friday the allegations about Trump's mental fitness were "absolutely outrageous".

They're "desperate attempts" to attack Trump, she said. "What's I think is really mentally unstable" is people not seeing the progress the president is making.

Wolff said he stands by everything in his book and has notes and recordings to back it up. While Trump said he never spoke to Wolff for the book, the author said he spoke to Trump both during the campaign and after the inauguration.

"Whether he realised it was an interview or not," the conversations weren't off the record, Wolff said.

'He's lost it'

Wolff said that his sources told him Trump repeats stories over an increasingly short time period, and that he sometimes doesn't recognise longtime friends.

NBC interviewer Savannah Guthrie asked Wolff what he was getting at.

Wolff quoted former Trump adviser Steve Bannon, who's undergone a dramatic public split with the president as the book has become public: "He's lost it."

Sanders also said Wolff never interviewed Trump about the book and has "made up a lot of stories" to try to sell books. Asked whether Bannon was the main person leaking information to Wolff, Sanders said Bannon "spent a lot more time with reporters than he ever did with the president."

Physical exam

Trump's campaign released a letter from his doctor in September 2016 saying he was in "excellent physical health." Trump plans to undergo a physical exam by a government doctor on Jan 12 that will be followed by a public statement by the doctor, the White House has said.

Past presidents have also undergone periodic exams by government physicians who released statements afterward. In an interview later Friday with NPR's All Things Considered, Wolff said "certainly it's an open discussion in the White House, as Steve Bannon put it, is he losing it?"