President Donald Trump threatened his former chief strategist Steve Bannon with legal action over “defamatory” statements about Trump's son and son-in-law over a meeting with Russians during the 2016 US presidential campaign.

Trump cut ties with Bannon on Wednesday, saying his former adviser had “lost his mind” in a blistering statement issued after reports of disparaging comments by Bannon in a forthcoming book, “Fire and Fury: Inside the Trump White House” by author Michael Wolff.

In the book, Bannon was quoted as describing a June 2016 meeting with a group of Russians at Trump Tower in New York as “treasonous” and “unpatriotic.” The meeting, held after the Russians promised damaging information on Democratic presidential candidate Hillary Clinton, was attended by Donald Trump Jr., Trump's son-in-law Jared Kushner and Paul Manafort, who was Trump's campaign manager at the time.

Lawyers for the president sent a cease-and-desist letter to Bannon on Wednesday. They said in the letter, seen by Reuters, that Bannon had breached an agreement by communicating with Wolff about Trump, his family and the campaign and made disparaging remarks about Trump and his family.

Trump personal lawyer Charles Harder said in a statement, “Legal action is imminent,” according to media reports.

The Washington Post reported on Thursday that Harder also sought to block Tuesday's publication of the book, which offers a lacerating portrait of an accidental presidency and a dysfunctional White House.

A legal notice to Wolff and publisher Henry Holt said Trump's lawyers were pursuing possible charges including libel, the Post reported. Harder did not immediately return a request for comment by Reuters.

On Thursday, the White House said no personal devices, including cellphones, would be allowed in the West Wing beginning next week for security purposes. The moves followed the Bannon split but had been considered for some time.

Trump had relied heavily on Bannon, chairman of the right-wing Breitbart News website, for advice in the months leading up to his upset victory in the November 2016 election. He then gave Bannon a strategic role in the White House, where he became a divisive figure before being fired in August. Bannon returned to Breitbart and continued to talk with Trump.

Trump's statement also diminished Bannon's role in the election victory and accused him of leaking to the media.

TRUMP'S LINK TO BASE

Bannon helped Trump shape a populist, anti-establishment message and had been the president's link to his hard-line conservative base of support. It was not clear if the split would push Bannon to be even more aggressive in his ongoing campaign against the Republican Party establishment, whether he would now target Trump or would emerge much weaker.

On Thursday, Bannon told a caller on Breitbart News radio that “nothing will ever come between us and President Trump and his agenda,” according to CNN.

The story that triggered the Trump-Bannon split was an offshoot of the investigation into whether Trump campaign aides colluded with Russia to sway the election to Trump, allegations Trump and Moscow deny.

According to the book, Bannon expressed derision about the June 2016 meeting in which a Russian lawyer offered damaging information about Clinton. Bannon also was quoted as saying he was sure Trump Jr. would have taken the Russians to meet his father in Trump Tower.

In the book, Bannon also said he believed the investigation into possible collusion between the Trump campaign and Moscow will focus on money laundering.

Special counsel Robert Mueller, who leads the Russia investigation, has asked White House officials to preserve records of the Trump Tower meeting, according to a source with knowledge of the request.

Manafort and business associate Rick Gates, another campaign aide, pleaded not guilty in November to federal charges brought by Mueller including conspiracy to launder money. Manafort sued Mueller on Wednesday, alleging that his investigation exceeds its legal authority.