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US President Donald Trump has commuted the prison sentence of his former adviser Roger Stone

The announcement came just after the Washington DC Court of Appeals denied Stone's request to delay the start date of his custodial term of 40 months.

He was convicted of lying to Congress, obstruction and witness tampering.

Stone was the sixth Trump aide found guilty on charges linked to a justice department probe that alleged Russia tried to boost the Trump 2016 campaign.

What did the president say?

The White House said in a statement: "Roger Stone is a victim of the Russia Hoax that the Left and its allies in the media perpetuated for years in an attempt to undermine the Trump Presidency."

It said that Department of Justice prosecutors under special counsel Robert Mueller only charged Stone out of frustration after failing to prove the "fantasy" that the Trump campaign had colluded with the Kremlin.

"This is why the out-of-control Mueller prosecutors, desperate for splashy headlines to compensate for a failed investigation, set their sights on Mr Stone," said the statement. 

The White House also suggested that the FBI had tipped off CNN about their pre-dawn raid on Stone's house, noting that a camera crew for the cable network was on the scene to record the arrest.

"Roger Stone has already suffered greatly," the statement said. "He was treated very unfairly, as were many others in this case.  Roger Stone is now a free man!" 

It's hard to say the president's decision to grant clemency to his long-time friend and political counsellor is surprising, but it's still jarring. Even though the president has called the special counsel investigation that prosecuted Stone a sham and a partisan witch hunt, Stone was duly convicted of serious crimes.

Meanwhile, Stone's active lobbying for a commutation, recently saying that he could have "easily" turned on the president to avoid trial, was unseemly at best.

Mr Trump is not the first president to issue controversial pardons or commutations for friends and associates, of course. Most of his predecessors, however, waited until the last days of their presidency to take such actions, as they knew the political firestorms they would generate.

Mr Trump, on the other hand, seems to relish the controversy. Like much of his time in office, his actions are done with an eye toward a base that views instigating political opponents as an end to itself.

While the action will be sharply criticised, at this point in the Trump presidency his critics and his allies appear pretty much set in stone. Giving Stone a reprieve won't win Mr Trump any new support, but that's not the point. He's helping a loyal friend, critics be damned.