A New York judge on Wednesday ordered Eric Trump, the president's son most involved with running the family business, to be deposed by 7 October as part of an investigation over charges that the organisation improperly inflated the value of real estate assets.

Letitia James, the Democratic attorney general of New York state who opened the inquiry, has been asking since May to question Eric Trump, 36, who has emerged as the helmsman of the Trump Organisation since his father moved to the White House in early 2017.

Even though Eric Trump has said he was ready to "cooperate" with the inquiry, his lawyers recently asked for the session to be pushed back until after the November elections, arguing that he was too busy, as he is heavily involved in his father's re-election campaign.

The attorney general had challenged that request in a court in the state, asking that Eric Trump be forced to provide documentation on several of the company's properties, including a building on Wall Street and the Trump International Hotel in Chicago.

After a court hearing on Wednesday, Judge Arthur Engoron ruled in favour of the attorney general, calling the younger Trump's arguments "unconvincing" and noting that "nor is this Court bound by the timelines of the national election".

"We will immediately move to ensure that Donald Trump and the Trump Organisation comply with the court's order and submit financial records related to our investigation," said James.

"The court's order today makes clear that no one is above the law, not even an organisation or an individual with the name Trump," she said.

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The investigation is one of several legal proceedings involving Trump and his family.

The attorney general opened the probe in 2019 after the president's former lawyer Michael Cohen, who has since been jailed, testified to Congress that Trump had inflated or under-reported the values of certain properties to secure loans or reduce his taxes.

In another investigation, Trump has been put on notice by Manhattan's attorney general, Cyrus Vance, after failing to provide eight years' worth of accounts.

Despite a decision that went against him in July in the Supreme Court, his lawyers are still fighting in the courts to avoid submitting those documents, and the case could head back to the Supreme Court again.