Robert Mueller has defended his investigation into Russia meddling in the 2016 US election and said Roger Stone “remains a convicted felon” despite his prison sentence being commuted by Donald Trump.

It comes as the US president’s intervention into Stone’s case was rebuked by Democrats and a few Republicans amid calls for an investigation.

Mr Trump defied the conventions of his office to commute the sentence of political confidant Roger Stone, four months ahead of US election day.

Mr Mueller wrote in the Washington Post that he was compelled to speak to “respond both to broad claims that our investigation was illegitimate and our motives were improper, and to specific claims that Roger Stone was a victim of our office”.

The investigation led to convictions for six Trump aides or advisers – including Stone, a larger-than-life political character who embraced his reputation as a dirty trickster.

Mr Mueller wrote: “The Russia investigation was of paramount importance. Stone was prosecuted and convicted because he committed federal crimes. He remains a convicted felon, and rightly so.”

Mr Trump commuted the sentence just days before Stone was due to report to prison to serve 40 months for crimes related to the Russian collusion investigation.

The White House confirmed the commuting of the sentence in a statement, saying Stone was a victim of the Russia “hoax”.

Though short of a full pardon, the commutation alarmed critics who have long been critical of the president’s repeated interventions in the nation’s justice system.

Stone had been sentenced in February for lying to Congress, witness tampering and obstructing the House investigation into whether the Trump campaign colluded with Russia to win the 2016 election.

But most of Republicans who did speak out about the decision supported it. Senator Lindsey Graham, a Trump confidant, said Stone was convicted of a “non-violent, first-time offence” and the president was “justified” in commuting the sentence.

Mr Trump likely cannot afford more political damage. He is trailing Joe Biden – according to his campaign’s own private admissions – and his effort to reboot his re-election bid took another blow when a planned rally for Saturday night in New Hampshire was postponed.