U.S. President Donald Trump’s former national security adviser Michael Flynn, who pleaded guilty to lying to the FBI, will be sentenced on Dec. 18, according to a court filing on Wednesday.

Mr. Flynn pleaded guilty in December, 2017, to lying to the FBI about his contacts with Russia, in exchange for co-operating with special counsel Robert Mueller’s probe into Russian interference in the 2016 presidential election.

A joint filing at U.S. District Court in Washington on Monday by Mr. Flynn’s lawyers and prosecutors had said Mr. Flynn could be sentenced as soon as Nov. 28.

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Prosecutors have repeatedly deferred Mr. Flynn’s sentencing date since his guilty plea, even though Mr. Flynn’s lawyer, Robert Kelner, said at a July hearing his client was eager to get the sentencing date on the calendar and put the ordeal behind him.

A sentencing date signals Mr. Flynn’s continuing co-operation with Mr. Mueller’s office is coming to an end.

Mr. Flynn was the first member of Trump’s administration to plead guilty to a crime uncovered by Mr. Mueller’s wide-ranging investigation into Russian attempts to influence the 2016 U.S. election and potential collusion by Trump aides.

Under a plea bargain deal, Mr. Flynn admitted in a Washington court that he lied when asked by FBI investigators about his conversations with Russia’s then-ambassador, Sergey Kislyak, just weeks before Mr. Trump took office.

Mr. Trump has denied knowing anything about contact with Russians and his campaign and has called Mr. Mueller’s probe a witch hunt. Russia has denied U.S. allegations that it interfered in the campaign.