US President Donald Trump has pardoned his former national security adviser Michael Flynn.

Mr Trump tweeted: “It is my Great Honor to announce that General Michael T. Flynn has been granted a Full Pardon. Congratulations to @GenFlynn and his wonderful family, I know you will now have a truly fantastic Thanksgiving!”

Mr Flynn is the second associate of Mr Trump’s convicted in the FBI’s investigation of his 2016 presidential election campaign’s ties with Russia to be granted clemency by the president.

Mr Trump commuted the sentence of long-time confidant Roger Stone just days before he was to report to prison. It is part of a broader effort to undo the results of an investigation that for years has shadowed his administration and yielded criminal charges against half a dozen associates.

The action voids the criminal case against Mr Flynn just as a federal judge was weighing up, sceptically, whether to grant a Justice Department request to dismiss the prosecution despite Mr Flynn’s own guilty plea to lying to the FBI about his Russia contacts.

The move, coming as Mr Trump winds down his single term, is likely to energise supporters who have rallied around the retired army lieutenant general as the victim of what they assert is an unfair prosecution.

Mr Trump himself has repeatedly spoken warmly about Mr Flynn, even though special counsel Robert Mueller’s prosecutors once praised him as a model co-operator in their probe into ties between Russia and the 2016 Trump campaign.

US President Donald Trump
US President Donald Trump (Susan Walsh/PA)

The pardon is the final step in a case defined by twists and turns over the last year after the Justice Department abruptly move to dismiss the case, insisting that Mr Flynn should have never been interviewed by the FBI in the first place, only to have US District Justice Emmet Sullivan refuse the request and appoint a former judge to argue against the federal government’s position.

In the months since, a three-judge panel’s decision ordering Mr Sullivan to dismiss the case was overturned by the full appeals court, which sent the matter back to Mr Sullivan.

At a hearing in September, Mr Flynn’s lawyer Sidney Powell told the judge that she had discussed Mr Flynn’s case with Mr Trump but also said she did not want a pardon – presumably because she wanted him to be vindicated in the courts.

Ms Powell emerged separately in recent weeks as a public face of Mr Trump’s efforts to overturn the results of his election loss to President-elect Joe Biden, but Mr Trump’s legal team ultimately distanced itself from her after she advanced a series of uncorroborated conspiracy claims.

The pardon spares Mr Flynn the possibility of any prison sentence, which Mr Sullivan could potentially have imposed had he ultimately decided to reject the Justice Department’s dismissal request. That request was made in May after a review of the case by a federal prosecutor who had been specially appointed by attorney general William Barr.

Mr Flynn acknowledged lying during the FBI interview by saying he had not discussed with the then-Russian ambassador, Sergey Kislyak, sanctions that had just been imposed on Russia for election interference by the outgoing Obama administration. During that conversation, Mr Flynn urged Mr Kislyak for Russia to be “even-keeled” in response to the punitive measures, and assured him “we can have a better conversation” about relations between the two countries after Mr Trump became president.