A US federal judge has dismissed the classified documents case against Donald Trump in what US media outlets reported was a huge victory for the former president.

Trump was accused of violating the Espionage Act. He was alleged to have retained classified national security documents at his home in Florida, including information regarding US nuclear programs and the capabilities of the US and other countries' militaries.

The Florida district judge on Monday dismissed the case, saying the appointment of Special Counsel Jack Smith to lead the investigation was unconstitutional, as his appointment was not approved by Congress.

The ruling made no mention of whether Trump's handling of documents was lawful or not.

US media outlets reported that the latest development marked the first time one of his four criminal cases has been dismissed entirely.

They also reported that prosecutors are likely to appeal the decision.

Trump -- who became the target of an assassination attempt during a rally on Saturday -- has called for the dismissal of other cases against him. He wrote on social media that, "As we move forward in Uniting our Nation after the horrific events on Saturday, this dismissal of the Lawless Indictment in Florida should be just the first step."