Ex-US President Donald Trump is suing his former lawyer, Michael Cohen, for $500m (£400m), alleging breach of contract.
He says Cohen breached his duty as attorney to act in his client’s best interests.
Cohen is a key witness in a New York investigation into Mr Trump, focusing on hush-money paid to a porn star.
He is “confident that the suit will fail based on the facts and the law,” Lanny Davis, Cohen’s spokesman and lawyer, told the BBC.
The lawsuit, 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.
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.”
“Mr Cohen will not be deterred and is confident that the suit will fail based on the facts and the law.”
The lawsuit comes amid escalating attacks on Cohen from Mr Trump’s circle, as the former president fends off a criminal case.
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 Mr Trump.
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.”