Michael Wolff defends book and says of Trump: 'To quote Steve Bannon: He's lost it'

• Author of Fire and Fury says he stands by ‘absolutely everything in the book’

• Fire and Fury released early in defiance of attempt to ban

• Fire and Fury: Inside the Trump White House review – tell-all burns all

The author of an explosive new book about Donald Trump’s first year in office has said he stands by his reporting and that the president’s threat of legal action is only boosting his sales.

Michael Wolff also stood by his account that many in the administration think Trump is incapable of meeting the demands of his role – claims which have prompted a furious backlash from the White House.

The author noted testimony that Trump keeps repeating himself: whereas once he would tell the same three stories in 25 or 30 minutes, he said, now he does so in 10.

“I will quote Steve Bannon,” Wolff said. “‘He’s lost it.’”

Discussing Trump’s response to the book on NBC’s Today show on Friday, in his first interview since publication of excerpts from book by the Guardian triggered a political firestorm, Wolff asked: “Where do I send the box of chocolates?

“Not only is he helping me sell books, but he’s helping me prove the point of the book.

“I mean, this is extraordinary that a president of the United States would try to stop the publication of a book. This doesn’t happen, has not happened from other presidents, would not even happen from the CEO of a mid-sized company.”

Trump’s lawyer demanded a halt to publication of Fire and Fury: Inside the Trump White House or excerpts. Its publisher instead brought forward the release date to Friday. The book offers a scathing portrait of a dysfunctional White House and a president out of his depth.

Trump attacked Wolff on Thursday evening, tweeting: “I authorized Zero access to White House (actually turned him down many times) for author of phony book! I never spoke to him for book. Full of lies, misrepresentations and sources that don’t exist.”

Wolff, a media critic and columnist, responded: “What was I doing there if he didn’t want me to be there? I absolutely spoke to the president. Whether he realised it was an interview or not, I don’t know, but it certainly was not off the record.”

The author spent about three hours with Trump during the election campaign and in the White House, he continued, “so my window into Donald Trump is pretty significant”.

He added: “But even more to the point, and this was really sort of the point of the book, I spoke to people who spoke to the president on a daily, sometime minute-by-minute basis. In a sense there was one question on my mind when I began this book: what is it like to work with Donald Trump, how can you work with Donald Trump, and how do you feel having worked with Donald Trump?”

Wolff’s new book is drawn from what he said was regular access to the West Wing and more than 200 interviews, including with Trump. Asked how he gained such extraordinary access, Wolff said: “I certainly said whatever was necessary to get the story.”

The White House has repeatedly dismissed the book as “tabloid gossip”, “sad”, “pathetic” and full of “lies” and some media commentators have cast doubt on Wolff’s credibility.

Wolff said that, like any journalist, he has recordings and notes and is “in every way comfortable” with everything he reported in the book.

“My credibility is being questioned by a man who has less credibility than perhaps anyone who has ever walked on earth at this point,” he said.

Pressed on whether he stands by everything in the book, Wolff replied: “Absolutely everything in the book.”

A recurring theme in Fire and Fury is the question of Trump’s fitness for office. Wolff suggested that even the president’s son-in-law, Jared Kushner, and daughter, Ivanka Trump, have lost faith in his ability to do the job.

“Certainly Jared and Ivanka, in their current situation, which is a deep legal quagmire, are putting everything on the president,” he said. “‘Not us, it’s him.’”

Wolff added: “I will tell you the one description that everyone gave, everyone has in common. They all say, ‘He is like a child,’ and what they mean by that is he has a need for immediate gratification. It’s all about him. He just has to be satisfied in the moment.

“They say he’s a moron, an idiot. Actually, there’s a competition to sort of get to the bottom line here of who this man is. Let’s remember, this man does not read, does not listen, so he’s like a pinball just shooting off the sides.”

The White House has pushed back against the claims of Trump’s incapacity, the press secretary, Sarah Sanders, saying on Thursday such claims were “disgraceful and laughable”.

On Friday, a longtime friend and confidant of Trump rejected Wolff’s suggestion that the president is losing mental stability.

Chris Ruddy, chief executive of conservative Newsmax Media, told CNN: “He is not psychologically unfit, he has not lost it, as [Wolff] claimed. I saw the president every other day over a 10-day period during the holiday. I conversed with him during this time. I saw him interact with people. He was remembering things, he was on point, he was following up on discussions.”

Ruddy recalled that he took Michael Schmidt, a New York Times journalist, to meet and interview the president at his Mar-a-Lago estate in Florida over Christmas.

“I don’t believe Michael walked out and said, ‘This man is crazy, this man is unfit’. So this is just an absurd allegation by someone who’s talked to a lot of disgruntled people at the White House.”

Ruddy added: ‘This is just an absurdity and it’s really trash, actually.”