Former US president Donald Trump is suing his former lawyer Michael Cohen for more than $US500 million ($747 million), according to a filing in federal court in Florida.

The lawsuit comes after Mr Cohen, once Mr Trump’s loyal “fixer,” testified before a Manhattan grand jury that later indicted Mr Trump on 34 felony counts of falsifying business records, marking the first time in US history that a former president has been charged with a crime.

Mr Trump, who is seeking the 2024 Republican presidential nomination, pleaded not guilty in that case on April 4.

Mr Cohen is poised to be a star witness in any eventual trial in the case, which centres around a hush money payment he made before the 2016 election to a porn star who says she slept with Mr Trump.

The lawsuit accuses Mr Cohen of violating his attorney-client relationship with Mr Trump by revealing his “confidences” and “spreading falsehoods” in books, podcasts and media appearances.

It says Mr Cohen wrongfully called Mr Trump “racist” in his 2020 book Disloyal and fabricated conversations with Mr Trump.

“The timing of Disloyal‘s release, just prior to the November 3, 2020 presidential election, suggests that defendant intended to improperly disclose plaintiff’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.

Mr Trump lost the election to Democrat Joe Biden.

Mr Cohen did not immediately respond to a request for comment.

Mr Cohen was a top executive at Mr Trump’s real estate company and then worked as his personal lawyer when Mr Trump assumed office in 2017.

While Mr Cohen said that year that he would “do anything” to protect Mr Trump, his stance had changed dramatically by 2019 when he testified before a US congressional committee.

“I am ashamed because I know what Mr Trump is,” Mr Cohen told the panel.

“He is a racist. He is a con man. He is a cheat.”

Mr Cohen in 2018 pleaded guilty to violating federal election law through the $US130,000 payment to the porn star, Stormy Daniels.

He was sentenced to three years in prison for that and other crimes, including cheating on his personal taxes and lying under oath to Congress about when the Trump Organisation stopped working on a proposed building project in Russia.

Mr Trump denies the affair with Ms Daniels.

He is accused by Manhattan District Attorney Alvin Bragg of falsifying Trump Organisation records to conceal that he reimbursed Mr Cohen for the payment to Mr Daniels.