A U.S. judge on Monday dismissed the criminal case accusing Donald Trump of illegally holding onto classified documents, dealing the former president another major legal victory in what some analysts believed was the most formidable case he was facing among his four criminal indictments.

Florida-based U.S. District Judge Aileen Cannon, who was nominated by Trump, ruled that special counsel Jack Smith, who is leading the prosecution, was unlawfully appointed to his role and did not have the authority to bring the case.

It marked another blockbuster legal victory for Trump, following the U.S. Supreme Court ruling on July 1 that as a former president he enjoyed immunity from prosecution for many of his actions in office. It comes two days after he survived an assassination attempt, and as he prepares to announce his vice-presidential running mate as the presumptive nominee at the Republican convention this week in Milwaukee.

Trump has been accused of taking thousands of papers containing some of the nation's most sensitive national security secrets when he left the White House in January 2021 and storing them in a haphazard manner at his Mar-a-Lago estate in Florida. Reportedly, another document may have been in Trump's possession at a New Jersey property he owned.

The 37-count indictment included violations of the Espionage Act, which criminalizes unauthorized possession of defence information, and conspiracy to obstruct justice, which carries a maximum sentence of 20 years in prison. The charges included references to dozens of top-secret or secret documents.

The U.S. Justice Department will appeal the ruling, a spokesperson for the special counsel said in a statement.

Courts in other cases have repeatedly upheld the ability of the U.S. Justice Department to appoint special counsel to handle certain politically sensitive investigations.

"This ruling flies in the face of about 20 years of institutional precedent, conflicts with rulings issued in both the Mueller investigation and in D.C. with respect to Jack Smith himself," said Bradley Moss, a lawyer who specializes in national security.

But Cannon's ruling throws the future of the case, which once posed serious legal peril for Trump, into doubt. Smith is also prosecuting Trump in federal court in Washington over his attempts to overturn the 2020 election, but the former president's lawyers have not made a similar challenge to the special counsel in that case.

The classified documents investigation was first referred to prosecutors in 2022 after the U.S. National Archives and Records Administration tried for more than a year to retrieve presidential records from Trump.

Following the judge's decision on Monday dismissing the documents case, Trump said his other outstanding prosecutions should also be thrown out. He is still awaiting trial on two cases — a federal prosecution in Washington and a Georgia state prosecution — for his attempts to overturn his 2020 election defeat.

Trump is also due to be sentenced in New York in September for trying to cover up a hush-money payment to porn star Stormy Daniels in the weeks before his 2016 election victory.

"This dismissal of the Lawless Indictment in Florida should be just the first step, followed quickly by the dismissal of ALL the Witch Hunts," Trump said on his Truth Social site on Monday, also referencing the prosecutions of hundreds of his supporters who stormed the U.S. Capitol on Jan. 6, 2021.

WATCH l Judge says classified documents case against Trump was unconstitutional:

Trump’s classified document charges dismissed

Former U.S. president Donald Trump scored a major legal victory when a judge dismissed the case against him for allegedly stealing and concealing classified U.S. government documents.

Congress approval needed, Trump lawyers argued

Trump's lawyers challenged the legal authority for Attorney General Merrick Garland's 2022 decision to appoint Smith to lead investigations into Trump. They argued the appointment violated the U.S. Constitution because his office was not created by Congress and he was not confirmed by the Senate.

Lawyers in Smith's office disputed Trump's claims, arguing there was a well-settled practice of using special counsel to manage politically sensitive investigations.

The ruling is the latest and most consequential in a series of decisions from Cannon favouring Trump's defence and expressing skepticism about the conduct of prosecutors. The judge previously delayed a trial indefinitely while considering a flurry of Trump's legal challenges.

In an unusual move, she allowed three outside lawyers, including two who sided with Trump, to argue during a court hearing focused on Trump's challenge to Smith's appointment.

Conservative Supreme Court Justice Clarence Thomas also provided a boost to Trump's challenge to the special counsel. In an opinion agreeing with the court's decision to grant Trump broad immunity in the election case, Thomas questioned whether Smith's appointment was lawful using similar arguments to those made by Trump's lawyers.

Garland appointed Smith, a public corruption and international war crimes prosecutor, to give investigations into Trump a degree of independence from the Justice Department under President Joe Biden's administration.

Two others — Trump personal aide Walt Nauta and Mar-a-Lago property manager Carlos De Oliveira — were also charged with obstructing the investigation.

The search of Trump's property in the summer of 2022 angered Republicans, and one Trump supporter was shot dead three days later after trying to enter an FBI office in Cincinnati.

The case also inspired searches among other high-profile politicians. Biden and former vice-president Mike Pence each returned documents that were located on properties they owned or managed as a result, with no criminal changes resulting.