U.S. President Donald Trump's trade adviser Peter Navarro knowingly and willfully violated federal law by attacking then-Democratic presidential candidate Joe Biden while serving in his official capacity, the U.S. Office of Special Counsel (OSC) said on Monday.

The federal watchdog agency explained its conclusion in a 13-page report to Trump, accusing Navarro of violating the Hatch Act on several occasions and noting that Navarro's "violations of the Hatch Act were knowing and willful."

The agency left it up the president however to determine what it called "appropriate disciplinary action."

"OSC's investigation determined that Dr. Navarro violated the Hatch Act's prohibition against using his official authority or influence to affect an election by engaging in political activity during official media appearances and on his official Twitter account," the agency said.

The Hatch Act, a U.S. federal law enacted in 1939, prohibits federal executive branch employees from using their official authority or influence for the purpose of interfering with or affecting the result of an election.

From May 26 through October 19, Navarro lambasted Biden, who won the November 3 presidential election, and his running mate Kamala Harris, during at least six media interviews in which he appeared in his official capacity, the report said.

The Office of Special Counsel said Navarro chose not to respond to the report. It said he continued to engage in the behavior despite being trained on the Hatch Act, and even after OSC launched its investigation.

The White House had no immediate comment.

Special Counsel Henry Kerner last year recommended that Trump fire adviser Kellyanne Conway after multiple violations of the Hatch Act, but he refused.

U.S. Representative Don Beyer, the Democratic vice chair of the Joint Economic Committee, told Reuters that while Navarro was unlikely to face consequences, a rank-and-file federal worker who engaged in similar behavior would likely have been fired.

In its report, OSC said the White House counsel's Office had claimed Navarro did not violate the law, and that his statements about Joe Biden "were the kind of factual, policy-based statements acceptable for him to make in his official capacity," OSC said.

But that was not the case, the watchdog said.

"A federal employee acting in his official capacity violates the Hatch Act when, as here, he makes statements intended to encourage others to vote for or against a candidate for partisan political office or otherwise promotes or disparages that candidate's campaign. Dr. Navarro violated the Hatch Act because he engaged in that very activity," it said.

(With input from Reuters, Xinhua)