Donald Trump warned a New York judge to “leave my children alone”, hours before his eldest son, Donald Jr, was due to take the stand in a $250 million civil fraud trial.

The lawsuit alleges years of financial misconduct by the former president and his two eldest sons, who are both executive vice-presidents of the Trump Organisation.

The case could deal a major blow to the family’s business empire, and Mr Trump’s reputation, as he campaigns to retake the White House in 2024.

The former president reacted with fury after three of his adult children were called to testify in the case, and accused Arthur Engoron - the judge overseeing it - of being a “partisan political hack”.

“Engoron is crazy, totally unhinged, and dangerous,” he wrote in a string of statements on his Truth Social platform on Wednesday.

“Leave my children alone, Engoron. You are a disgrace to the legal profession!”

Mr Trump also said he would appeal a gag order imposed by Judge Engoron, who has twice fined him for breaching it.

Don Jr, 45, is the first of Mr Trump’s adult children to be called to testify. He has cultivated a political profile in his own right, regularly acting as his father’s attack dog, railing against his political foes and fiercely defending him in the media.

Don Jr’s testimony is expected to mark another flashpoint in a trial that has oscillated between hours of tedious testimony over financial statements to testy exchanges between the defence and the prosecution.

His younger brother, Eric, 39, could testify as early as Thursday. The former president and his daughter Ivanka, 42, are due to testify next Monday and Wednesday respectively.

Ivanka left the Trump Organisation in 2017 to advise her father in the White House and was removed as a defendant in the case after a judge ruled her involvement exceeded the statute of limitations.

However, she has still been compelled to testify in the case. Her testimony is likely to take at least a day.

The case has been brought by Letitia James, New York’s attorney general, who watched from the public gallery on Wednesday.

Ms James has alleged the Trump Organisation and its executives fraudulently inflated their worth by up to $2.2 billion (£1.8 billion) from 2011 to 2021 in order to secure more favourable loan rates and insurance premiums.

The Trump family and their company - which encompasses a global real estate portfolio including residential and office buildings, luxury hotels and golf courses - have denied any wrongdoing.

They have argued that subjective valuations of the group’s worth, including assets such as Trump Tower in Manhattan, were made in good faith and that the banks lending to them did not lose any money.

“They all made money, and there is no Victim (except me!)” Mr Trump repeated on Truth Social on Wednesday.

Mr Trump’s mounting legal woes have not dented his commanding polling lead over his Republican rivals in the race to claim the party’s presidential nomination.

Unlike Mr Trump’s four criminal cases, the New York fraud case does not carry the threat of jail time.

However, it carries more immediate risks to the Trump family, a point underscored by Mr Trump’s frequent presence in court.

Judge Engoron has already determined that Mr Trump and the Trump Organisation “overstated assets” in order to obtain bigger loans secured against them. But the trial will determine if other violations of financial laws occurred and what fine should be imposed.

Ms James is seeking a $250 million (£206 million) fine and a ban on Mr Trump and his sons working as executives in New York.

‘Trump too small’

Meanwhile on Wednesday, America’s highest court heard arguments in a dispute over an attempt to trademark the phrase “Trump too small”.

Steve Elster, a California lawyer and progressive activist, sought to sell T-shirts featuring the phrase after a former Republican rival mocked the size of Mr Trump’s hands on TV.

“You know what they say about guys with small hands,” Marco Rubio, a Florida senator and former Republican presidential candidate, said during a 2016 presidential debate.

Mr Trump responded by saying: “If they’re small, something else must be small. I guarantee you there is no problem.”

Mr Elster’s 2018 application to register the phrase “Trump too small” was rejected by the US trademark office, which concluded Mr Trump’s written consent was required.

An Appeals Court disagreed, finding the decision violated Mr Elster’s right to criticise political figures under free speech protections.

The case was escalated to the US Supreme Court by the federal government, which argued Mr Elster could not assert exclusive rights “in another person’s name”.