NEW YORK, United States (AP) — A judge ruled Tuesday that Donald Trump committed fraud for years while building the real estate empire that catapulted him to fame and the White House.

Judge Arthur Engoron, ruling in a civil lawsuit brought by New York Attorney General Letitia James, found that the former president and his company deceived banks, insurers and others by massively overvaluing his assets and exaggerating his net worth on paperwork used in making deals and securing financing.

Engoron ordered that some of Trump's business licences be rescinded as punishment, making it difficult or impossible for them to do business in New York, and said he would continue to have an independent monitor oversee the Trump Organization's operations.

Trump's lawyer and spokesperson Alina Habba said they intend to appeal the decision, calling it "an affront to our legal system" and "fundamentally flawed at every level".

Trump has long insisted he did nothing wrong. His son, Eric, railed against the decision Tuesday in a post on X, formerly known as Twitter, calling it "an attempt to destroy my father and kick him out of New York".

"Today, I lost all faith in the New York legal system," said Eric Trump, an executive in his father's company and a defendant in the lawsuit. "Never before have I seen such hatred toward one person by a judge — a coordinated effort with the Attorney General to destroy a man's life, company and accomplishments."

The decision, days before the start of a non-jury trial in James' lawsuit, is the strongest repudiation yet of Trump's carefully coiffed image as a wealthy and shrewd real estate mogul turned political powerhouse.

Beyond mere bragging about his riches, Trump, his company and key executives repeatedly lied about them on his annual financial statements, reaping rewards such as favourable loan terms and lower insurance costs, Engoron found.

Those tactics crossed a line and violated the law, the judge said, rejecting Trump's contention that a disclaimer on the financial statements absolved him of any wrongdoing.

"In defendants' world: rent regulated apartments are worth the same as unregulated apartments; restricted land is worth the same as unrestricted land; restrictions can evaporate into thin air; a disclaimer by one party casting responsibility on another party exonerates the other party's lies," Engoron wrote in his 35-page ruling. "That is a fantasy world, not the real world."

Manhattan prosecutors had looked into bringing criminal charges over the same conduct but declined to do so, leaving James to sue Trump and seek penalties that aim to disrupt his and his family's ability to do business in the state.

Engoron's ruling, in a phase of the case known as summary judgement, resolves the key claim in James' lawsuit, but several others remain. He'll decide on those claims and James' request for US$250 million in penalties at a trial starting October 2. Trump's lawyers have asked an appeals court for a temporary delay.