Former White House aide Steve Bannon "lost his mind" after he lost his job, US President Donald Trump has said.
The president disavowed Bannon after he was quoted in a new book describing a meeting between Trump's son and a group of Russians as "treasonous".
According to Bannon, the Russians had offered Donald Trump Jr damaging information on Hillary Clinton at the June 2016 meeting.
Trump's lawyers have sent a cease-and-desist letter to Bannon, US media reports said.
"Steve Bannon has nothing to do with me or my presidency. When he was fired, he not only lost his job, he lost his mind," the BBC quoted Trump as saying in a statement on Wednesday.
"Steve was a staffer who worked for me after I had already won the nomination by defeating seventeen candidates, often described as the most talented field ever assembled in the Republican party," he continued.
"Now that he is on his own, Steve is learning that winning isn't as easy as I make it look. Steve had very little to do with our historic victory, which was delivered by the forgotten men and women of this country."
Trump's lawyers said Bannon had broken his employment agreement by speaking to the book's author Michael Wolff about Trump and his family, "disclosing confidential information" and "making disparaging statements and in some cases outright defamatory statements to Wolff about Trump, his family members, and the Company," the letter said, according to the Washington Post newspaper.
Bannon, the president's former chief strategist, was considered a key player in the Trump White House and helped shape Trump's "America First" campaign message before he left his post in August.
He returned to his role as the head of the right-wing Breitbart News website, where he said he planned to help Trump's administration as a "wingman outside".
Trump reportedly spoke to Bannon as recently as 13 December, the day of the special US Senate election in Alabama that saw the defeat of Republican Roy Moore, whom Bannon supported.
The president's comments came hours after the explosive new book, Fire and Fury: Inside the Trump White House, revealed that Bannon called Trump Jr's meeting "treasonous" and "unpatriotic".
Speaking to the author, Bannon said of the investigation into whether there was any collusion between Russia and the Trump campaign: "They're going to crack Don Junior like an egg on national TV."
