Donald Trump’s ex-fixer inflated the valuation of the former president’s assets to “whatever number Mr Trump told us to”, he has testified. Michael Cohen made the allegation as he took the stand at Trump’s civil fraud trial in New York. His highly anticipated testimony is at the heart of the prosecutors’ case against his former boss. Cohen – the trial’s star witness – claimed he was “tasked, by Mr Trump, to increase the total assets based upon a number that he arbitrary selected”, in an effort to magnify his net worth and “obtain better insurance premiums”. Trump, who attended the trial on Tuesday, dismissed Cohen as a “proven liar” and told reporters he was “not worried at all” about his former lawyer’s testimony. Judge Arthur Engoron has already ruled Trump and his family business committed fraud. Engoron is using this trial – focused on remaining claims of conspiracy, insurance fraud and falsifying business records – to decide on punishment. “This is not about Donald Trump v Michael Cohen, or Michael Cohen v Donald Trump,” Cohen said as he arrived at court in Manhattan. “This is about accountability, plain and simple.” The two men have not been in the same room since Cohen, Trump’s personal attorney for more than a decade, turned on his boss. The former executive vice-president at the Trump Organization ultimately pleaded guilty in 2018 to felony charges, including tax evasion and lying to Congress during an investigation of Trump’s ties with Russia. Trump continued to attack Cohen before entering the courtroom on Tuesday. “He’s a proven liar, as you know, a felon,” the former president told reporters. “We did nothing wrong and that’s the truth.” Cohen’s congressional testimony in 2019 – during which he alleged that Trump “inflated his assets when it served his purposes” – led the office of the New York attorney general, Letitia James, to pursue its fraud case against the former president. As prosecutors laid out their case during opening arguments earlier this month, a video clip was briefly shown in court which showed Cohen telling investigators that Trump “wanted to be higher on the Forbes list”, and had directed him to increase the value of assets to “accommodate” a higher net worth. Cohen, 57, was once a staunchly loyal Trump employee before breaking away from his former boss in 2018, and pleading guilty to federal charges. These included lying to Congress and facilitating illegal hush-money payments. He was sentenced to three years in prison, mostly spent in home confinement. Since his release in November 2021, Cohen has been a vocal critic of Trump. “It appears that I will be reunited with my old client,” he wrote on X, formerly Twitter, ahead of his appearance. He posted a meme featuring both himself and the former president, with the caption: “Let’s get you back to your cell.” He has already released one book, Disloyal, in which he brands his former boss “a cheat, a liar, a fraud, a bully, a racist, a predator and a conman”, and is preparing to release a second which accuses Trump of having “weaponized the Department of Justice” to pursue his critics. Trump has been incensed by Cohen’s decision to break with his business empire, describing his former fixer as a “rat” and even trying to sue him for $500m for “spreading falsehoods” with “malicious intent”, before dropping the lawsuit. The trial is a bench trial, with no jury. Engoron is presiding over the case, and will be the sole decider. Because this is a civil trial, Trump will not be sent to prison if found guilty. He is not required to appear in court. Trump, 77, has repeatedly and vehemently protested his innocence, claiming that the case against him amounts to a “fraud” and a political witch-hunt. The trial has been shrouded by acrimony, and early on Trump shared a social media post attacking Engoron’s clerk and identifying her by name. The judge subsequently barred the parties from speaking publicly about court staff. While Trump deleted the post, last week Engoron said that a screenshot had remained live on the former president’s campaign site for weeks. The judge, who acknowledged that the lapse seemed “inadvertent”, fined Trump $5,000 and warned that future violations would bring “far more severe” sanctions, such as imprisonment.