Donald Trump was intimately involved with all aspects of a scheme to stifle stories about sex that threatened to torpedo his 2016 campaign, his former lawyer said on Monday in matter-of-fact testimony that went to the heart of the former president’s hush money trial.

“Everything required Mr Trump’s sign-off,” said Michael Cohen, Trump’s fixer-turned-foe and the prosecution’s star witness in a case now entering its final, pivotal stretch.

In hours of highly anticipated testimony, Cohen placed Trump at the centre of the hush money plot, saying the then-candidate had promised to reimburse the lawyer for the money he fronted and was constantly updated about behind-the-scenes efforts to bury stories feared to be harmful to the campaign.

“We need to stop this from getting out,” Cohen quoted Trump as telling him in reference to porn actor Stormy Daniels’ account of a sexual encounter with Trump a decade earlier. The then-candidate was especially anxious about how the story would affect his standing with female voters.

A similar episode occurred when Cohen alerted Trump that a Playboy model was alleging that she and Trump had an extramarital affair. “Make sure it doesn’t get released,” was Cohen’s message to Trump, the lawyer said. The woman, Karen McDougal, was paid $150,000 in an arrangement that was made after Trump received a “complete and total update on everything that transpired.”

“What I was doing, I was doing at the direction of and benefit of Mr Trump,” Cohen testified.

Trump has pleaded not guilty and denied having sexual encounters with the two women. Cohen is by far the prosecution’s most important witness, and although his testimony lacked the electricity that defined Daniels’ turn on the stand last week, he nonetheless linked Trump directly to the payments and helped illuminate some of the drier evidence such as text messages and phone logs that jurors had previously seen.

The testimony of a witness with such intimate knowledge of Trump’s activities could heighten the legal exposure of the presumptive Republican presidential nominee if jurors deem him sufficiently credible. But prosecutors’ reliance on a witness with such a checkered past – Cohen pleaded guilty to federal charges related to the payments – also carries sizeable risks with a jury and could be a boon to Trump politically as he fundraises off his legal woes and paints the case as the product of a tainted criminal justice system.

The men, once so close that Cohen boasted that he would “take a bullet” for Trump, had no visible interaction inside the courtroom. The sedate atmosphere was a marked contrast from their last courtroom faceoff, when Trump walked out of the courtroom in October after his lawyer finished questioning Cohen during his civil fraud trial.

This time around, Trump sat at the defence table with his eyes closed for long stretches of testimony as Cohen recounted his decade-long career as a senior Trump Organization executive, doing work that by his own admission sometimes involved lying and bullying others on his boss’s behalf.

Jurors had previously heard from others about the tabloid industry practice of “catch-and-kill,” in which rights to a story are purchased so that it can then be quashed. But Cohen’s testimony, which continues on Tuesday, is crucial to prosecutors because of his direct communication with the then-candidate about embarrassing stories he was scrambling to suppress.

Cohen also matters because the reimbursements he received from a $130,000 hush money payment to Daniels, which prosecutors say was meant to buy her silence in advance of the election, form the basis of 34 felony counts charging Trump with falsifying business records. Prosecutors say the reimbursements were logged, falsely, as legal expenses to conceal the payments’ true purpose. Defence lawyers say the payments to Cohen were properly categorized as legal expenses.

Under questioning from a prosecutor, Cohen detailed the steps he took to mask the payments. When he opened a bank account to pay Daniels, an action he said he told Trump he was taking, he told the bank it was for a new limited liability corporation but withheld the actual purpose.

“I’m not sure they would’ve opened it,” he said, if they knew it was ”to pay off an adult film star for a non-disclosure agreement.”

To establish Trump’s familiarity with the payments, Cohen told the jury that Trump had promised to reimburse him. The two men even discussed with Allen Weisselberg, a former Trump Organization chief financial officer, how the reimbursements would be paid as legal services over monthly instalments, Cohen testified.

Although Trump’s lawyers have said he acted to protect his family from salacious stories, Cohen described Trump as preoccupied instead by the impact they would have on the campaign.

• An AP report