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Trump's remarks came after Britain's The Guardian newspaper earlier on Wednesday revealed that Bannon, quoted from excerpts of a new book, described the 2016 meeting at Trump Tower between Trump's son and campaign aides and a Russian lawyer as "treasonous" and "unpatriotic."
The former executive has also described President Donald Trump's daughter Ivanka Trump as "dumb as a brick" in the upcoming book, 'Fire and Fury'.
According to the Guardian, the book is based on hundreds of interviews, including the ones taken from Trump's courtiers.
"Steve Bannon has nothing to do with me or my presidency," Xinhua news cited Trump as saying, "When he was fired, he not only lost his job, he lost his mind."
"Steve was rarely in a one-on-one meeting with me and only pretends to have had influence to fool a few people with no access and no clue, whom he helped write phony books," the president said.
"Steve pretends to be at war with the media, which he calls the opposition party, yet he spent his time at the White House leaking false information to the media to make himself seem far more important than he was," he added.
"Steve does not represent my base -- he's only in it for himself," Trump said in his statement.
Following Trump's statement, White House press secretary Sarah Huckabee Sanders fiercely slapped Wolff's book, which is said to draw on more than 200 interviews.
"This book is filled with false and misleading accounts from individuals who have no access or influence with the White House," Sanders was quoted as saying.
"Participating in a book that can only be described as trashy tabloid fiction exposes their sad, desperate attempts at relevancy."
The Bannon's observation about Ivanka was reported on Wednesday, by The Wall Street Journal, which has reviewed Michael Wolff's new book.
Bannon also reportedly told Wolff that investigation into the Trump campaign's potential ties to Russia is centered on money laundering and claimed that "they're going to crack Don Junior like an egg on national TV".
Bannon was chief executive of the Trump campaign in its final three months during the 2016 US elections.
He became the White House chief strategist after Trump took office in January 2017 and left the White House in August before returning to his perch as chairman of Breitbart News.