Donald Trump's former campaign adviser, George Papadopoulos, who has also been an outspoken critic of Alexander Downer and the Australian Government, has lost his bid to avoid prison.

District Court Judge Randolph Moss sided with US Special Counsel Robert Mueller to reject Papadopoulos' last-ditch attempt to avoid starting a 14-day sentence.

Papadopoulos was ordered to surrender himself on Monday (local time) to a Wisconsin prison.

"The court agrees with the Special Counsel that Papadopoulos has failed to carry his burden of demonstrating that a delay in the execution of his sentence is warranted," Mr Moss wrote.

The prison in Oxford, Wisconsin, is medium-security and used to house first-time, non-violent offenders.

Papadopoulos' partner, Simona Mangiante, said she would support him when he went behind bars.

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Papadopoulos had asked the judge to delay the sentence until separate court proceedings challenging the constitutionality of Mr Mueller's appointment were decided.

In the lead-up to his prison sentence, Papadopoulos used social media and multiple media interviews to allege — without evidence backing it up — that Mr Downer, former Australian high commissioner to the UK, and the Australian and UK governments were spying on him and were trying to undermine Mr Trump.

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Papadopoulos pleaded guilty on October 5, 2017 to one count of making false statements to the FBI about his interactions with "a female Russian national" and Joseph Mifsud, a Maltese professor whom Papadopoulos "understood to have significant ties to the Russian Government".

Papadopoulos claimed that in May 2016, five months before the US presidential election, Mr Downer was spying on him when they met for gin and tonics at a London bar.

Papadopoulos will spend 14 days in a medium-security Wisconsin prison. ( Reuters: Yuri Gripas )

Mr Downer, who has denied the spying claims, told The Australian newspaper Papadopoulos said during their bar meeting: "The Russians might use material that they have on Hillary Clinton in the lead-up to the election, which may be damaging."

Papadopoulos, 31, from Chicago, said he could not remember telling Mr Downer the volatile information that would eventually spark the FBI probe into Russian interference in the 2016 election.

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Mr Trump has had mixed words about Papadopoulos, who was little-known before the future president announced in an interview before the election he was "an excellent guy" and part of his foreign policy team.

After Papadopoulos was sentenced earlier this year, Mr Trump told reporters: "I don't know Papadopoulos."

Former Trump campaign adviser Michael Caputo also downplayed Papadopoulos' significance to Mr Trump's election bid by describing him merely as a "coffee boy".

"You might have called him a foreign policy analyst, but if he was going to wear a wire, all we would have known now is whether he prefers a caramel macchiato over a regular American coffee in conversations with his barista," Mr Caputo told CNN.

AAP