The prosecutor who led lengthy investigations into U.S. President Joe Biden's son defended his efforts in a report on Monday, saying the criminal charges against Hunter Biden were the culmination of "thorough, impartial" work and "not partisan politics."

The report from special counsel David Weiss also criticized the senior Biden for pardoning his own son and scolded him for questioning the integrity of the criminal cases in a December statement, in which the president said he believed his son had been treated "differently" on account of his last name.

"Other presidents have pardoned family members, but in doing so, none have taken the occasion as an opportunity to malign the public servants at the Department of Justice based solely on false accusations," the report from Weiss read Monday.

"These baseless accusations have no merit and repeating them threatens the integrity of the justice system as a whole," the prosecutor continued.

"The president's characterizations are incorrect based on the facts in this case and, on a more fundamental level, they are wrong."

Hunter Biden was convicted on three felonies in June after he lied on a federal form to purchase a gun in Delaware by saying he wasn't a drug user in 2018, a period when he has acknowledged being addicted.

The president went back on previous pledges when he issued his son a federal pardon on Dec. 1. The move sparked a political uproar in Washington, drawing criticism from Democrats as well as Republicans and threatening to cloud the politician's legacy as he prepares to leave office on Jan. 20.