Former National Security Adviser Michael Flynn is firing his attorneys, as he awaits sentencing in his criminal case.

According to a motion filed in D.C. federal court on Thursday, Flynn is ending his relationship with the law firm Covington & Burling, and has already hired new legal representation. The motion was filed by Covington lawyers Robert Kelner and Stephen Anthony, who asked the court for permission to formally withdraw from the case.

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"General Flynn has notified the undersigned that he is terminating Covington & Burling LLP as his counsel and has already retained new counsel for this matter," the motion said.

Flynn pleaded guilty to providing false statements to the FBI during a January 2017 interview. He admitted to lying about his communications with Russian ambassador Sergey Kislyak in December 2016.

Flynn has been cooperating for months with former Special Counsel Robert Mueller and federal prosecutors, and this move could signal he may be contesting his upcoming sentencing -- and perhaps may wish to change his plea.

Flynn has consistently asked the federal judge in this case, Emmet Sullivan, to delay his sentencing while he continued his government cooperation. Flynn attended a sentencing hearing in December 2018.

That hearing featured dramatic exchanges between Flynn team and the judge. Flynn's lawyers had previously called into question "the circumstances of the FBI interview" in a memorandum seeking a non-jail sentence. Sullivan asked Flynn if he wanted to withdraw his guilty plea, giving him multiple chances to do so or to challenge the circumstances of his FBI interview.

Flynn declined, but Sullivan delayed sentencing. The judge has yet to set a sentencing date.

Sources close to the matter say the push for Flynn to retain new counsel began following that hearing, and accelerated with reports that Flynn had already been under investigation when he had his December 2016 calls with Kislyak. The defense was unaware of this when Flynn entered his plea.

Flynn's change in strategy was also inspired by news that when the Trump campaign had an intelligence briefing in August 2016, there was no warning of Russian outreach or of any investigation of Flynn, the sources said.

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In May, Sullivan ordered federal prosecutors to release a transcript of recorded conversations between Flynn and Kislyak, but prosecutors refused, stating the transcript was not relevant in ascertaining that Flynn was guilty or in making a sentencing recommendation.

Sullivan appeared to be satisfied with this, stating in a notice on the court's docket system, "Upon consideration of the government’s submissions in response to those orders, the government is not required to file any additional materials or information on the public docket."

Fox News' Catherine Herridge contributed to this report.