A US judge appointed by Donald Trump has dismissed charges against the former President, accusing him of illegally keeping classified documents when he left office.

Judge Aileen Cannon ruled lead prosecutor Special Counsel Jack Smith was unlawfully appointed - and didn't have the authority to bring the case.

Channel Nine US Correspondent Lauren Tomasi says such a ruling initially seemed far-fetched.

"It was a bit of a longshot that his legal team had put in, thinking it would never come to fruition."

Trump's posted on Truth Social, saying he's very pleased about the outcome.

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