US President Donald Trump's lawyers have written to his former strategist Steve Bannon, saying he has violated a non-disclosure agreement.
The cease-and-desist notice accuses Mr Bannon of defaming the president in speaking to author Michael Wolff.
Wolff's forthcoming tell-all book makes several startling claims, and questions Mr Trump's fitness for office.
The president responded by saying Mr Bannon had "lost his mind" after losing his White House position.
His lawyers said Mr Bannon had broken his employment agreement by speaking to Wolff about Mr Trump, his family and the campaign, "disclosing confidential information" and "making disparaging statements and in some cases outright defamatory statements".
On his radio show on Wednesday, Mr Bannon responded to the president's criticism by saying he was a "great man".
"You know, I support him day in and day out," he said on the show produced by right-wing Breitbart News, which he heads.
The book is reportedly based on more than 200 interviews.
On Thursday, the Hollywood Reporter published Wolff's behind-the-scenes account. He concluded that everyone he had spoken to was in agreement in their view on Mr Trump: "They all - 100 percent - came to believe he was incapable of functioning in his job".
What's in the book?
Fire and Fury: Inside the Trump White House makes many explosive claims, including that:
- Mr Bannon thought a meeting between Donald Trump Jr and a group of Russians was "treasonous"
- The Trump team was shocked and horrified by his election win
- His wife, Melania, was in tears on election night
- Mr Trump was angry that A-list stars had snubbed his inauguration
- The new president "found the White House to be vexing and even a little scary"
- His daughter, Ivanka, had a plan with her husband, Jared Kushner, that she would be "the first woman president"
- Ivanka Trump mocked her dad's "comb-over" hairstyle and "often described the mechanics behind it to friends"
It also alleges that former UK Prime Minister Tony Blair told Mr Trump during a meeting last February that the British intelligence services may have been spying on him and his campaign, according to a report in The Times newspaper.
The Times says Mr Blair was hoping to get a job advising Mr Trump on the Middle East.
Mr Blair told the BBC Today programme the story was "complete fabrication, from beginning to end". He said he had discussed the Middle East peace process with Mr Kushner but denied angling for work.
Who is Michael Wolff?
The 64-year-old writer is a former columnist for New York magazine and Vanity Fair.
According to New York magazine, which first published the extracts, Wolff was able to exploit the Trump administration's political inexperience to gain an unusual amount of insight.
Wolff said he was able to take up "something like a semi-permanent seat on a couch in the West Wing" following the president's inauguration.
However, some of the book's excerpts have already been criticised and questioned. The Washington Post said Wolff was a "provocateur and media polemicist", and that his reporting had been questioned before.
How has the Trump administration defended itself?
"Steve Bannon has nothing to do with me or my presidency," Mr Trump said in a statement on Wednesday.
Press secretary Sarah Sanders dismissed the book as a "trashy tabloid fiction" that she said was "filled with false and misleading accounts from individuals who have no access or influence with the White House".
A spokesperson for Melania Trump said the First Lady had encouraged her husband's presidential bid. "She was confident he would win and was very happy when he did," she said on Wednesday.
What's being said in Bannon's defence?
Aside from the radio comment, Mr Bannon has not spoken publicly about the book, the president's comments or the legal action.
However, in an interview with BBC Newsnight, Raheem Kassam - UK editor of Breitbart News - said the Wolff interviews could have been taken out of context.
He also said Mr Bannon was well-placed to comment on the Russia controversy, having served in the US Navy.
"He understands what the geopolitical threat is..." said Mr Kassam. "So when he remarks that these people shouldn't probably have been in Trump Tower taking meetings with senior campaign staff without lawyers in the room, I would say that is a fair thing to say."
An inevitable break-up
Anthony Zurcher, BBC News, Washington
Donald Trump swept to the presidency in part on the back of Steve Bannon and his Breitbart conservative media empire. Now we will see how he fares when he is at war with them.
The president's blistering reply to Mr Bannon's comments appears to indicate that the bridge between the politician and his ideological spirit guide has been reduced to cinders. But how will Mr Trump's legion of supporters react? It is never wise to underestimate their dedication to the man himself, above all else.
No matter the outcome of this coming battle, this has to be viewed as a devastating failure for Mr Bannon personally. After spending years advocating for an anti-establishment conservative populism, he finally had a seat in the halls of power. He said in early 2017 that his goal was nothing short of the "deconstruction of the administrative state".
Now he is on the outside again, besieged by long-time antagonists and former allies. His president recently signed a tax bill embraced by corporate interests. His first post-2016 foray into elective politics, the Alabama Senate race, ended in humiliating defeat.
Perhaps, given all this, the Bannon-Trump feud was as inevitable as it is certain to be vicious.