A former US lawmaker who was Donald Trump's first pick to run the Justice Department paid for sex numerous times, including with an underage girl, according to a congressional report released Monday.

Matt Gaetz also regularly used cocaine and ecstasy, and bought marijuana from his Capitol Hill office, according to the 37-page document, the culmination of a long-running probe by the House Ethics Committee.

"The committee determined there is substantial evidence that Representative Gaetz violated House rules and other standards of conduct prohibiting prostitution, statutory rape, illicit drug use, impermissible gifts, special favours or privileges, and obstruction of Congress," panel investigators wrote, according to reports.

Gaetz has repeatedly denied wrongdoing - pointing to the Justice Department's decision not to bring charges against him in 2023 after a criminal probe.

The report came out despite him suing the committee to block its release.

Congressional investigators found that the 42-year-old broke multiple Florida laws on sexual misconduct, although they cleared him of federal sex trafficking violations.

The report listed payments by Gaetz totalling more than US$90,000 to 12 women "likely in connection with sexual activity and/or drug use" between 2017 and 2020.

The ex-congressman was a staunch Trump loyalist and a favourite of the president-elect's ardent supporters.

He resigned from Congress in November after Trump nominated him for US attorney general. (AFP)