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WASHINGTON (Reuters) - President Donald Trump’s former deputy campaign chairman Rick Gates, who gave crucial testimony as a prosecution witness that helped convict former Trump advisers Paul Manafort and Roger Stone, will be sentenced on Dec. 17 in his own criminal case, a court filing showed on Friday.

Gates, who had initially pleaded not guilty to charges brought by former Special Counsel Robert Mueller, reversed himself in February 2018 and pleaded guilty to charges including conspiracy against the United States and lying to investigators.

As part of his plea deal, Gates agreed to cooperate with Mueller’s investigation, which documented Russian interference in the 2016 U.S. presidential election. Gates faces a maximum prison term of nearly six years when he is sentenced.

His testimony was an important part of the prosecution case last year against Manafort, Trump’s 2016 campaign chairman and Gates’ former business partner. Manafort’s lawyers tore into Gates on the witness stand, prompting him to admit to an extramarital affair and having embezzled money. But Manafort was convicted by a jury in federal court in Virginia of tax and bank fraud and is now serving a prison sentence of 7-1/2 years.

He also appeared as a prosecution witness in federal court in Washington against Stone, the longtime Trump adviser and veteran Republican operative who was convicted by a jury on Friday on seven counts of lying to the U.S. Congress, obstruction and witness tampering. [nL2N27V0S8]

In pivotal testimony, Gates told jurors on Tuesday that he witnessed a phone call between Trump and Stone related to the WikiLeaks website in late July 2016.

In a court filing on Monday, prosecutors disclosed that the testimony by Gates in the Stone trial would conclude his cooperation with the government, and they asked a judge to set a sentencing date.

Gates, a former political consultant and lobbyist, is one of several former Trump aides to have pleaded guilty or been convicted of charges brought by Mueller, who completed his investigation in March. In addition to his senior role as a campaign adviser, Gates also worked for Trump’s presidential inaugural committee before Trump took office in January 2017.