A New York judge has ruled that former US president Donald Trump and two of his adult children must answer questions under oath in the state attorney-general’s civil probe into their family company’s business.

Justice Arthur Engoron of New York state court in Manhattan ruled in favour of Attorney-General Letitia James, who sought to enforce subpoenas to compel testimony by Mr Trump, his son Donald Trump Jr and his daughter Ivanka Trump.

Justice Engoron said Ms James had “the clear right” to question the Trumps after having uncovered “copious evidence of possible financial fraud.”

Justice Engoron directed the Trumps to submit to questioning within 21 days.

Alan Futerfas, a lawyer for the younger Trumps, declined to comment. Lawyers for Donald Trump did not immediately respond to requests for comment.

The decision followed a two-hour hearing in which the Trumps’ lawyers accused Ms James of doing an end run around their clients’ constitutional rights by seeking testimony she could them use against them in a parallel criminal probe.

Donald Trump’s lawyer Alina Habba accused Ms James of “selective prosecution and prosecutorial misconduct that this country has never seen”, citing statements reflecting the Democratic attorney-general’s “vile disdain” for Mr Trump.

“If he was not who he is, she would not be doing this,” Ms Habba said. “This court can help stop this circus.”

Kevin Wallace, a lawyer from Ms James’ office, rejected that characterisation.

“They haven’t shown anything here that says it’s unfair,” Mr Wallace said.

Last month, Ms James said her nearly three-year investigation into the Trump Organisation had uncovered significant evidence of possible fraud.

She described what she called misleading statements about the values of the “Trump Brand” and six Trump properties, saying the company may have inflated real estate values to obtain bank loans and reduced them to lower tax bills.

The investigation partially overlaps a criminal probe now led by Manhattan District Attorney Alvin Bragg in which the Trump Organisation and its long-time chief financial officer pleaded not guilty last July to tax fraud charges.

Ms James joined that probe last May.

Mr Trump has called Ms James’ investigation a political “witch hunt” and is suing to try to stop it.

The hearing followed last week’s decision by Mr Trump’s long-time accounting firm Mazars USA to cut ties with him and the Trump Organisation, saying it could no longer stand behind a decade of financial statements.

-AAP