Robert Costello will raise questions about the account of key witness Michael Cohen in the hush money case, reports say.

Robert Costello has been asked to testify on Monday after he said he had information that raises questions about the credibility of Michael Cohen, a key witness in the investigation into alleged hush money payments made to an adult film star during the 2016 election campaign, the reports said.

Citing unidentified sources, the Reuters and Associated Press news agencies reported that Costello will challenge Cohen’s account of the events.

Costello told The Washington Post newspaper that he has an “ethical obligation to report” what he knows. He will testify behind closed doors, and the details of his statements, as well as whether his appearance will have an effect on the investigation, remain unclear.

The testimony comes two days after Trump said he expected to face criminal charges in relation to the case and urged his supporters to protest his possible arrest.

The New York probe centres on a $130,000 payment that Cohen, Trump’s former personal lawyer and fixer, said was made to porn star Stormy Daniels in the waning days of the former president’s 2016 campaign. Prosecutors say the funds amount to an illegal campaign donation.

Daniels has claimed that she had sexual relations with Trump, who is married, an allegation that the former president denies.

Cohen was sentenced to three years in prison in 2018 over his payments to Daniels and another woman, as well as for lying to Congress. US law requires disclosing campaign contributions and limits them to $2,700 per individual.

Trump denied early on that the payment to Daniels was connected to the campaign, saying that the agreement with the porn star, whose real name is Stephanie Clifford, “was used to stop the false and extortionist accusations made by her about an affair”.

And in recent days, the former president – who is seeking the White House again in 2024 – has repeatedly blasted Alvin Bragg, an elected Democratic district attorney overseeing the investigation in New York.

Trump said on Monday that the Daniels case is based on a “discredited” testimony by Cohen, describing his former fixer as a “convicted liar, felon and jailbird”.

“ALVIN BRAGG SHOULD BE HELD ACCOUNTABLE FOR THE CRIME OF ‘INTERFERENCE IN A PRESIDENTIAL ELECTION’,” Trump wrote on his Truth Social platform.

Many Republicans have come to Trump’s defence against the possible charges, with US House Speaker Kevin McCarthy accusing Bragg of “political vengeance”.

Responding to Trump’s weekend call for protests should he be arrested, however, McCarthy said he did not think people should take to the streets. “We want calmness out there,” he told reporters.

Beyond the New York case, Trump is facing federal investigations over his role in the January 6, 2021 attack on the US Capitol by a mob of his supporters, as well as his possible mishandling of classified documents.

The state of Georgia also is conducting a probe into Trump’s efforts to overturn the results of the 2020 election that he lost to President Joe Biden.

Trump has denied any wrongdoing in all the cases, dismissing them as politically motivated.