Former US president Donald Trump must answer questions under oath in New York state’s civil investigation into his business practices, a judge has ruled.

Judge Arthur Engoron ordered Mr Trump and his two eldest children, Ivanka and Donald Trump Jr, to comply with subpoenas issued in December by New York Attorney General Letitia James.

Mr Trump and his two children must sit for a deposition within 21 days, Judge Engoron said.

He issued the ruling after a two-hour hearing with lawyers for the Trumps and Ms James’ office.

“In the final analysis, a State Attorney General commences investigating a business entity, uncovers copious evidence of possible financial fraud, and wants to question, under oath, several of the entities’ principals, including its namesake. She has the clear right to do so.” Judge Engoron wrote in his decision.

Capitol Riot Investigation Ivanka Trump
Ivanka Trump must also answer questions, a judge said (Morry Gash/AP)

The ruling is almost certain to be appealed, but if upheld it could force the former president into a tough decision about whether to answer questions, or stay silent, citing his Fifth Amendment right against self-incrimination.

Ms James, a Democrat, said her investigation has uncovered evidence Mr Trump’s company used “fraudulent or misleading” valuations of assets like golf courses and skyscrapers to get loans and tax benefits.

Mr Trump’s lawyers told Judge Engoron during the hearing that having him sit for a civil deposition now, while his company is also the subject of a parallel criminal investigation, is an improper attempt to get around a state law barring prosecutors from calling someone to testify before a criminal grand jury without giving them immunity.

“If she wants sworn testimony from my client, he’s entitled to immunity. He gets immunity for what he says, or he says nothing,” Mr Trump’s criminal defence lawyer Ronald Fischetti said in the hearing, which was conducted by video conference.

If Mr Trump were to testify in the civil probe, anything he says could be used against him in the criminal investigation being overseen by the Manhattan district attorney’s office.

Mr Trump could invoke his Fifth Amendment right to remain silent in a deposition — something he has criticised others for doing in the past. But Mr Fischetti said if Trump did so, it could still hurt a potential criminal defence.

“If he goes in and follows my advice, which will be you cannot answer these questions without … immunity because that’s what the law provides, and take the Fifth Amendment, that’ll be on every front page in the newspaper in the world. And how can I possibly pick a jury in that case?” Mr Fischetti said.

A lawyer for the attorney general’s office, Kevin Wallace, told the judge that it was not unusual to have civil and criminal investigations proceeding at the same time.

“Mr Trump is a high profile individual, yes. That’s unique,” Mr Wallace said. “It’s unique that so many people are paying attention to a rather dry hearing about subpoena enforcement. But the legal issues that we’re dealing with here are pretty standard.”