Ex-US President Donald Trump is suing his former lawyer for $500m (£400m), alleging breach of contract.
He says Michael Cohen breached his duty as attorney to act in his client's best interests.
The lawsuit comes amid escalating attacks from Trump allies on Cohen, who is a key witness in a New York investigation into the ex-president.
A Manhattan prosecutor last week charged Mr Trump with fraud in relation to hush money payments to a porn star.
Cohen's spokesman and lawyer, Lanny Davis, told the BBC he was confident the lawsuit against his client would fail.
The legal action, filed in a Florida federal court, also accuses Cohen of making "improper, self-serving, and malicious statements about his former client, his family members, and his business".
Cohen worked as Mr Trump's attorney for over a decade. He was also a vice-president at the Trump Organization, and was often described as Mr Trump's fixer.
But the two had a major falling out after the 2016 election, as investigators began looking into several of Mr Trump's aides.
In 2018, Cohen was sentenced to three years in prison and a fine after pleading guilty to charges of fraud and campaign finance violations.
Now out of prison, Cohen has become a high-profile critic of Mr Trump, and a frequent guest on news programmes.
He has written a book and hosts a podcast, both of which Mr Trump cites in the lawsuit.
It says Cohen fabricated conversations and wrongfully called Mr Trump a "racist" in his 2020 book Disloyal.
Cohen's lawyer, Mr Davis, said in a statement to the BBC: "Mr Trump appears once again to be using and abusing the judicial system as a form of harassment and intimidation against Michael Cohen.
"It appears he is terrified by his looming legal perils and is attempting to send a message to other potential witnesses who are co-operating with prosecutors against him."
New York prosecutors have charged Mr Trump with 34 felony counts of falsifying business records, in what they say was an effort to cover up payments intended to keep a porn star, Stormy Daniels, quiet about her alleged affair with him.
On 4 April, Mr Trump appeared in Manhattan criminal court - the first former US president ever indicted on criminal charges. He pleaded not guilty.
Cohen has admitted that as Trump's fixer, he facilitated a $130,000 pay-out to Ms Daniels.
As Mr Trump's court date approached, Cohen made numerous appearances on major network news programmes and criticised his former boss.
"He's not thick-skinned," Cohen told CNN after Mr Trump's indictment. "He's actually very thin-skinned, and he has a very fragile ego."
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