Robert Mueller, the special prosecutor in charge of the Russia election meddling probe, recommended on Tuesday that President Donald Trump's former national security advisor Michael Flynn face no jail time due to his "substantial" cooperation with the investigation.

Mueller said in a court filing that Flynn, who admitted last year to lying about his contacts with Russians following  Trump's November 2016 election victory, had helped in his and other unspecified federal criminal investigations, including being interviewed 19 times.

Mueller also told the Washington Federal court that despite his "serious" offense, the retired three-star general and former Pentagon intelligence chief had a strong record of military and public service.

The surprise recommendation came ahead of Flynn's upcoming sentencing, which had been postponed four times over the past year.

Those postponements indicated that, once hostile to the investigation that threatens Trump and his inner circle, he had possibly become a valuable witness.

"Given the defendant's substantial assistance and other considerations set forth below, a sentence at the low end of the guideline range —including a sentence that does not impose a term of incarceration —is appropriate and warranted," Mueller said in a memorandum to the court.

Flynn's was the first guilty plea secured by the Mueller investigation into alleged collusion between Trump's election campaign and Russia.

His position as a top-level insider in the campaign, accompanying Trump to key events, making a keynote speech at the July 2016 Republican convention, and then taking hold of the White House national security apparatus in Trump's first weeks in office, made him a potentially extremely valuable witness.

But many thought his own problems had likely compromised that value.