The lawsuit accuses Michael Cohen of breaking a confidentiality agreement and ‘spreading falsehoods’ about the former president.

Donald Trump has sued his ex-lawyer Michael Cohen, seeking at least $500m in damages, as the former US president steps up attacks on his one-time loyal “fixer” after Cohen tested before the Manhattan grand jury that indicted Trump.

The lawsuit accused Cohen of breaking a confidentiality agreement he signed as a condition of his employment, violating ethical standards for lawyers and “spreading falsehoods” about Trump “with malicious intent and to wholly self-serving ends”.

The suit, filed in Miami on Wednesday, offered a preview of arguments that are sure to be featured in Trump’s defense against charges that he falsified internal business records to disguise payments made during the 2016 presidential campaign to silence claims of extramarital sexual encounters.

Trump is not specifically suing Cohen over his grand jury testimony in the criminal case, but he cites it in support of an argument that his ex-lawyer sought to profit from his role through the publication of two books, a podcast series and media appearances.

Cohen’s spokesman, lawyer Lanny Davis, said the lawsuit will not deter Cohen’s cooperation with prosecutors.

“Mr Trump appeared once again to be using and abusing the judicial system as a form of harassment and intimidation against Michael Cohen,” Davis said. “It appears he is terrified by his looming legal perils and is attempting to send a message to other potential witnesses who are cooperating with prosecutors against him.”

The suit is the latest effort by Trump to use the legal system to go after his political enemies and is another example of the former president turning on a once-loyal aide after their relationship was imploded.

It comes as Cohen, who once said he would “do anything” to protect Trump, appears poised to become a star witness against him at a possible criminal trial in New York on the charges unsealed last week. Trump, who is planning to run for the 2024 Republican presidential nomination, pleaded not guilty to 34 counts of falsifying business records. It marked the first time a former US president was charged with a crime.

Prosecutors led by Manhattan District Attorney Alvin Bragg, a Democrat, have said Trump covered up his reimbursement of Cohen for $130,000 in hush money paid before the 2016 elections to porn star Stormy Daniels, who has said she had a sexual encounter with Trump in 2006. Trump denies any such relationship.

Trump’s lawsuit said Cohen wrongfully called Trump “racist” in the disbarred lawyer’s 2020 book, Disloyal, and fabricated conversations with Trump from when he served as his lawyer.

“The timing of Disloyal’s release, just prior to the November 3, 2020 Presidential Election, suggests that (Cohen) intended to improperly disclose (Trump’s) confidences when it would be most lucrative to do so – and while Disloyal would be sure to have the most damaging reputational effect,” the lawsuit said.