Describing Trump’s move as “inappropriate and deeply regrettable”, they insisted that “we have never before seen the approval or removal of security clearances used as a political tool, as was done in this case.”

Signing the letter Thursday was a virtual who’s who of American spy chiefs dating back to the late 1980s, a striking show of solidarity from the top ranks of the national security establishment.

They included former directors of central intelligence William Webster, George Tenet and Porter Goss; former CIA Directors Michael Hayden, Leon Panetta and David Petraeus; former Director of National Intelligence James Clapper; and former deputy CIA directors John McLaughlin, Stephen Kappes, Avril Haines, David Cohen and Michael Morell.

Robert Gates, the former CIA director and secretary of defence, signed on to the letter Friday morning. Having served Republican and Democratic presidents, Gates is known for staying out of the political arena. His addition to the bipartisan list only served to underscore the alarm in national security circles following Trump’s punitive swipe at Brennan.

Two of those signing the statement – Clapper and Hayden – have been cited by Trump as on a list of people who could also lose their clearance.

Former intelligence officials often retain security clearance after leaving office to allow their successors to consult them on issues of the day.

After the White House said that Brennan – a notable Trump critic – had been stripped of clearance due to his “erratic” behaviour, Trump acknowledged that it was actually because of Brennan’s criticism and comments about the 2016 Trump campaign’s ties to Russia.

In an interview with The Wall Street Journal, Trump admitted that his decision on Brennan’s clearance was linked to the federal investigation, led by special counsel Robert Mueller, into possible collusion between his campaign and Russia to influence the 2016 election.

“I call it the rigged witch hunt, [it] is a sham,” Trump was quoted as saying in the interview. “And these people led it!” He added: “So I think it’s something that had to be done.”

Trump’s actions have been widely condemned, including by the respected retired admiral William McRaven, who oversaw the US Navy SEAL raid that killed Osama bin Laden in 2011.

“Through your actions, you have embarrassed us in the eyes of our children, humiliated us on the world stage and, worst of all, divided us as a nation,” he wrote Thursday in an op-ed article for The Washington Post.

The strongly worded response of typically measured and apolitical intelligence and military officials stood in stark relief to that of senior Republicans in Congress. Many said that Brennan deserved to have his clearance revoked in light of his unrelenting criticism of the president.

On Friday, Trump said he would likely remove the security clearance of Justice Department official Bruce Ohr, who has been targeted by supporters of the president, in part because his wife worked for a company that produced a dossier alleging Russia had incriminating evidence against Trump.

“Bruce Ohr is a disgrace. I suspect I’ll be taking it away very quickly,” Trump told reporters before leaving the White House for a trip to New York.

Trump said he had gotten a “tremendous response” since revoking Brennan’s clearance.

He also rejected the notion that he was restricting Brennan’s right to free speech.

“There’s no silence,” he said. “If anything, I’m giving him a bigger voice.”