Former President Donald Trump has reportedly filed an "emergency" lawsuit against a New York judge's gag order in his $250 million fraud trial.

Law360 correspondent Frank Runyeon reported that Trump had filed the suit with an appellate court.

In the filing, Trump accuses New York Supreme Court Justice Arthur Engoron of bias against him.

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"The Gag Orders unconstitutionally infringe on Petitioners' freedom of speech guaranteed by the First Amendment," Trump's legal team argued.

"Justice Engoron may not, by judicial decree, transmogrify the court's summary contempt power into an unfettered license to inflict public punishments on a defendant for the defendant's out-of-court statements made for the benefit of the fourth estate. Nor does the court's summary contempt power authorize Supreme Court and its staff to actively and independently investigate and prosecute violations of a gag order."

The motion said Engoron's gag order, "Casts serious doubt on his ability to function as an impartial finder of fact in a bench trial."

Under the limited gag order, Trump is prohibited from referring to the judge's staff. His attorneys are subject to a similar order.