Former CIA director John Brennan testifies before the House Intelligence Committee: Reuters

Former CIA Director John Brennan has issued a fiery rebuke to Donald Trump's controversial decision to revoke his access to the nation’s top secrets, while calling the president's claims of no collision with Russia during the 2016 election "hogwash".

Mr Brennan, who led the CIA for most of Barack Obama's second term, said that the president's actions – an unprecedented move in American history – were "politically motivated" and "an attempt to scare into silence others who might dare to challenge him".

The former intelligence official said that it was "critically important" that Special Counsel Robert Mueller be permitted to complete his investigation into Russian election meddling and possible collusion with Trump campaign officials. Mr Trump has repeatedly called the probe a "witch hunt".

Mr Brennan, a fierce critic of the president, wrote in the New York Times: "Mr Trump clearly has become more desperate to protect himself and those close to him."

"While I had deep insight into Russian activities during the 2016 election, I now am aware – thanks to the reporting of an open and free press – of many more of the highly suspicious dalliances of some American citizens with people affiliated with the Russian intelligence services," he added.

The former CIA director said that the “only questions that remain are whether the collusion that took place constituted criminally liable conspiracy" and "whether obstruction of justice occurred to cover up any collusion or conspiracy". Mr Brennan also suggested that it must be investigated whether "members of ‘Trump Incorporated’ attempted to defraud the government by laundering and concealing the movement of money into their pockets.”

This action is part of a broader effort by Mr. Trump to suppress freedom of speech & punish critics. It should gravely worry all Americans, including intelligence professionals, about the cost of speaking out. My principles are worth far more than clearances. I will not relent. https://t.co/TNzOxhP9ux — John O. Brennan (@JohnBrennan) August 15, 2018

"Mr. Trump’s claims of no collusion are, in a word, hogwash," Mr Brennan said.

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Ahead of the opinion piece, Mr Brennan had tweeted his displeasure via the medium used most by the president. "This action is part of a broader effort by Mr Trump to suppress freedom of speech & punish critics... It should gravely worry all Americans, including intelligence professionals, about the cost of speaking out," Mr Brennan wrote after the decision, made last month, was made public. "My principles are worth far more than clearances. I will not relent."

Mr Trump made the decision to remove Mr Brennan’s security clearance in a virtually unilateral move, bypassing the typical practice of having the FBI conduct security clearance reviews and making assessments on current and former officials who maintain access to classified intelligence.

"The issue of Mr. Brennan's security clearance raises larger questions about the practice of former officials maintaining access to our nation's most sensitive secrets long after their time in government has ended," the president said in a statement announcing the decision. "Such access is particularly inappropriate when former officials have transitioned into highly partisan positions and seek to use real or perceived access to sensitive information to validate their political attacks."

It’s typical for the former heads of the intelligence community to preserve their eligibility for clearances in order to advise new leadership on international developments and maintain continuity between administrations, among other reasons. Their ability to access information is only granted on a need-to-know basis, however, and would require the backing of a government agency or contracting company.

Mr Trump said on Wednesday that he is also reviewing the security clearances of nine other former officials, including former FBI director James Comey, who the president fired last year leading to the appointment of Mr Mueller. All of the nine on the list have criticised the president or been targeted by congressional Republicans seeking to discredit the Russia probe.

“I call it the rigged witch hunt,” Mr Trump told the Wall Street Journal about the the Russia probe and the revocation of Mr Brennan's security clearance . “And these people led it. So I think it’s something that had to be done.”

As for Mr Brennan, he has said he will not be silenced and that Mr Trump still has questions to answer. He wrote that the president's call as a candidate in July 2016 for Russia to help find missing emails of his Democratic opponent, Hillary Clinton, who was being investigated for having used a private email server while secretary of state.

“Such a public clarion call certainly makes one wonder what Mr Trump privately encouraged his advisers to do – and what they actually did – to win the election,” Mr Brennan said.