(CNN) Over a dozen former senior intelligence officials -- a group that includes officials who served under both Democratic and Republican presidents -- are denouncing President Donald Trump's decision to revoke former CIA Director John Brennan's security clearance as "ill-considered" and "unprecedented."

With the exception of Brennan -- and excluding those who served under Trump -- every CIA director since George Tenet, who took over the agency in 1996, signed the letter.

"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 former senior intelligence officials, including former CIA directors, former CIA deputy directors, and a former director of national intelligence, said in a statement released Thursday.

They went on to argue, "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," the former officials said, adding that the signal "is inappropriate and deeply regrettable."

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