Facebook reported BBC journalists to the police after they provided, under request, sexualised images of children discovered on the social network's private groups.

The episode occurred as part of a BBC investigation into Facebook's content moderation systems, which the news organisation says isn't effective.

The BBC reported dozens of photos to Facebook, including an apparent freeze frame showing child abuse, but more than 80 percent of the photos weren't removed.

Following the investigation, the BBC asked Facebook for an interview about its moderation system.

Its director of policy, Simon Milner, agreed to be interviewed "on condition the BBC provided examples of the material that it had reported, but had not been removed by moderators."

After the BBC did so, Facebook reported it to the UK's National Crime Agency.

The social network defended itself in a statement, saying it's "against the law for anyone to distribute images of child exploitation."

"When the BBC sent us such images we followed our industry’s standard practice and reported them to CEOP (Child exploitation & Online Protection Centre). We also reported the child exploitation images that had been shared on our own platform. This matter is now in the hands of the authorities," it said in a statement.

The chairman of the Commons media committee, Damian Collins, said this was "extraordinary" because the BBC was trying to help Facebook "clean up their network."