Those statements were made to Michael Wolff, the author of a book that chronicles Trump's first year in office. The president's attorney Charles Harder sent a letter to Bannon on Wednesday, demanding him to "cease and desist" any more of such claims.
In the book "Fire and Fury: Inside the Trump White House," Bannon was quoted as saying that a June 2016 Trump Tower meeting between the president's eldest son, Donald Jr., and a Russian lawyer was "treasonous" and "unpatriotic."
Bannon also said there was "zero" chance that then-candidate Trump did not meet with the attendees at a later date or time.
The book will hit shelves on Jan. 9 but excerpts of key sections were published on Wednesday.
Trump, in a Wednesday statement, blasted Bannon. He said the former chief strategist had "not only lost his job" but also "lost his mind" when he left the White House. The president added that Bannon had little to do with his election victory and blamed him for the loss in the Alabama special U.S. senate election last month.
Bannon was fired from the Trump administration in August last year as part of a broader purge of some of the most hardline nationalist voices in the West Wing. He has made some of the harshest criticisms about the president, such as calling the firing of former FBI Director James Comey the "biggest mistake in modern political history."
On Thursday, Harder sent a cease and desist letter to Wolff and his publisher, Henry Holt & Co., and demanded an apology for the content of the book.
— CNBC's Jacob Pramuk contributed to this report.