Twelve former CIA chiefs have denounced Donald Trump's decision to revoke John Brennan's national security clearance, condemning the move as "an attempt to stifle free speech".

The US president on Wednesday removed the ability for Mr Brennan – himself a former CIA director – to access sensitive government information, citing the Russia investigation as the reason behind the move.

But in an extraordinary joint statement on Thursday evening, the former US security officials said they felt "compelled" to respond to the "unprecedented" decision by the Mr Trump, whose allegations against Mr Brennan they branded "baseless".

"We all agree that the president's action regarding John Brennan and the threats of similar action against other former officials has nothing to do with who should and should not hold security clearances – and everything to do with an attempt to stifle free speech," the statement said.

It continued: "We have never before seen the approval or removal of security clearances used as a political tool, as was done in this case. Beyond that, this action is quite clearly a signal to other former and current officials.

"As individuals who have cherished and helped preserve the right of Americans to free speech – even when that right has been used to criticise us – that signal is inappropriate and deeply regrettable. Decisions on security clearances should be based on national security concerns and not political views."

John Brennan has attacked Donald Trump on Twitter (REUTERS)

The signees included seven former CIA directors, five former deputy directors and former director of national intelligence James Clapper. Two of the signees — Mr Clapper and former CIA director Michael Hayden — have appeared on a White House list of people who may also have their security clearances revoked.

Michael McFaul, former US ambassador to Russia, hailed the statement as "a big deal indeed".

Mr Trump openly tied his decision to strip Mr Brennan of his clearance to the ongoing investigation into Russian election interference and possible collusion with his campaign.

"I call it the rigged witch hunt, (it) is a sham," Mr Trump told The Wall Street Journal on Wednesday. "And these people led it!"

"So I think it's something that had to be done."

However, the connection to special counsel Robert Mueller's investigation was not mentioned in an earlier Trump statement released by the White House.

In the initial statement, the president denounced Mr Brennan's criticism of him and spoke of "the risks posed by his erratic conduct and behaviour." Mr Trump added he was fulfilling his "constitutional responsibility to protect the nation's classified information".

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The statement was dated 26 July upon its release on Wednesday, which prompted some to suggest it had been decided weeks in advance and but was saved to use as a political distraction. Moments later, a second statement was released with the date removed.

Writing in The New York Times, Mr Brennan said Mr Trump's decision to deny him access to classified information was a desperate attempt to end Mr Mueller's investigation. Mr Brennan, who served under Barack Obama and has become a vocal critic of the current administration, called Mr Trump's claims he did not collude with Russia "hogwash."

The only question remaining is whether the collusion amounts to a "constituted criminally liable conspiracy," Mr Brennan wrote.

Earlier on Wednesday, William H McRaven, the retired Navy admiral who oversaw the raid that killed Osama bin Laden, called Mr Trump's moves "McCarthy-era tactics", adding he would "consider it an honour" if Mr Trump revoked his clearance as well.

"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," McRaven wrote in an opinion piece for The Washington Post.

Attorneys said the revocation appeared to be within the president's authority, but they noted the power play could also be used to reinforce a case alleging obstruction of justice, following the president's firing of former FBI director James Comey and his repeated tweets calling for the investigation to end.

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Patrick Cotter, a former assistant US attorney in the Eastern District of New York and a longtime white collar defence attorney, said that while a prosecutor could argue Mr Trump's targeting of clearances was intended as a warning that "if you contribute to, participate in, support the Russia probe and I find out about it, I'm going to punish you," it is likely not obstruction in itself.

But, he said, the move would be a "powerful piece of evidence" for prosecutors as part of a pattern to demonstrate an intent to use presidential power in connection with the probe.

Renato Mariotti, a former federal prosecutor agreed.

"What it shows is that the president is fixated on the Russia investigation, he's angry about it, and he wants to do everything he can to discourage or slow down the investigation," he said.

Mr Mueller and his team have been looking at Mr Trump's public statements and tweets as they investigate whether the president could be guilty of obstruction.

"I don't think it advances the criminal obstruction case, but I think it's factually relevant," said Mark Zaid, a national security attorney. "I think it shows the state of mind and intent to interfere or impede any unfavourable discussion of his potential connection to Russia."

Former CIA directors and other top national security officials are typically allowed to keep their clearances, at least for some period. But Mr Trump said on Wednesday he is reviewing the clearances of several other former top intelligence and law enforcement officials, including Mr Comey and current senior Justice Department official Bruce Ohr. All are critics of the president or are people who Mr Trump appears to believe are against him.

"You look at any of them and you see the things they've done," Mr Trump told the WSJ. "In some cases, they've lied before Congress. The Hillary Clinton whole investigation was a total sham."

"I don't trust many of those people on that list," he said. "I think that they're very duplicitous. I think they're not good people."

The episode was reminiscent of Mr Trump's shifting explanations for firing Mr Comey and the evolving descriptions of the Trump Tower meeting between top campaign aides and a Kremlin-connected lawyer – both topics of interest to Mr Mueller.

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And it underscores why the president's lawyers are fearful of allowing Mr Trump to sit down for an interview with Mr Mueller's team, as Mr Trump has repeatedly said he is interested in doing.

In announcing Mr Comey's firing, the White House initially cited the former FBI director's handling of the probe into Democratic rival Hillary Clinton's emails, seizing on the FBI director's decision to divulge details of the probe to the public during her campaign against Mr Trump.

But a few days after Mr Comey was dismissed, Mr Trump told NBC's Lester Holt in an interview that he was really thinking of "this Russia thing" when he fired Mr Comey.

Mr Trump later changed direction again, tweeting that he "never fired James Comey because of Russia!"

Early this month, he admitted in a tweet that the Trump Tower meeting, which was arranged by his son, Donald Trump Jr, "was a meeting to get information on an opponent".

That directly contradicted a July 2017 statement from Mr Trump Jr – written with the consultation of the White House – that claimed the meeting had been primarily about adoption.