In a 43-page opinion, US District Judge Emmet Sullivan says that while he was required by law to throw out the case, he was not persuaded by many of the arguments that prosecutors previously made in urging him to dismiss it.

"The court finds both stated rationales dubious to say the least, arguably overcoming the strong presumption of regularity that usually attaches to prosecutorial decisions," Sullivan wrote.

A retired army general, Flynn pleaded guilty to lying to the FBI about interactions he had had with Russia's ambassador to the United States in the weeks before Trump's inauguration in January 2017.

He later tapped Sidney Powell, a lawyer pursuing so-far unsuccessful attempts to overturn the election in Trump's favour, and sought to withdraw the plea on the grounds that prosecutors violated his rights and duped him into a plea agreement.

His sentencing was deferred several times.

In May, Attorney-General William Barr abruptly intervened in the case and ordered the Justice Department to reverse course and dismiss the charge.

Prosecutors argued Flynn's statements to the FBI in his interview were not "material", that Flynn had a "faulty memory" and they no longer felt they had enough evidence for a conviction.

Sullivan skewered those arguments in his opinion, saying he felt the Justice Department's claims did not meet the legal standard for him to dismiss the case.

Australian Associated Press