A joint investigation unit for the National Women's Soccer League (NWSL) in the United States and their players association (NWSLPA) found widespread, ongoing misconduct in the top-flight North American league, according to a report released on Wednesday.

The unit received reports of ongoing misconduct at "more than half" of the NWSL clubs during the 14-month inquiry, after The Athletic initially outlined allegations of abuse in the league.

"The underlying culture of the NWSL created fertile ground for misconduct to go unreported," said Covington & Burling LLP and Weil, Gotshal & Manges LLP, the investigative team.

"Institutions meant to investigate and address misconduct failed to do so effectively."

The findings were broadly in line with the results of another investigation by former US Deputy Attorney General Sally Yates and King & Spalding LLP, who were brought in by the sport's national governing body, US Soccer.

The Yates report said in October that the NWSL and US Soccer failed to put in place "basic measures" to safeguard players.

"US Soccer, the League, and individual clubs were not clear on which entity held the ultimate responsibility for establishing and enforcing policies and practices relating to misconduct," the NWSL and NWSLPA joint investigation unit said.

US Soccer said in a statement that it was "carefully reviewing the report."

"We are grateful to the NWSL and NWSLPA for their important efforts to understand the factors that led to abuse in women's professional soccer and to identify meaningful steps to ensure player safety moving forward," the organisation said.

"US Soccer has closely communicated and cooperated with the NWSL and NWSLPA throughout their investigation. We share a common goal of ensuring a safe, healthy environment for players."

NWSL Commissioner Jessica Berman, who took the reins in April, said the joint investigation showed the league systematically failed to protect players.

"They deserve, at a minimum, a safe and secure environment to participate at the highest level in a sport they love," she said in a statement.

"We are committed to making all the necessary changes to create a safe and positive environment for our players, staff and fans."

The NWSL said it planned to make several changes, including hiring a player safety officer in the league and providing training for players and coaches, along with team and league staff.