Facebook is facing mounting criticism for failing to punish users caught discussing and sharing sexual images of children on its platform and – when confronted on the issue – for reporting the journalists who found such content to the UK police.

A BBC investigation found that out of 100 reported images, some of which contained images of under-16s in explicit poses and commentators talking about child abuse material, more than 80% were not taken down by Mark Zuckerberg's tech giant.

Other offending material uncovered by the BBC included secret groups with names including "hot xxxx schoolgirls" which consisted of stolen images of child victims and one picture that appeared to be a still from real abuse footage.

Out of 100 images, 18 were removed.

Despite Facebook having written policies on posting or discussing child abuse images, the BBC found – and reported – five convicted paedophiles with profiles on the platform. It revealed that even in this instance, none were taken down or censored.

The network's director of policy, Simon Milner, was set to be interviewed on the subject however not only pulled out of the request, but his firm reported news of the BBC investigation to a division of the UK National Crime Agency (NCA).

"One can only assume that the Facebook executives were unwilling or certainly reluctant to engage in an interview or a debate about why these images are available on the Facebook site," said David Jordan, the BBC's director of editorial policy in response.

Later, in a statement, Milner told IBTimes UK: "We have carefully reviewed the content referred to us and have now removed all items that were illegal or against our standards. This content is no longer on our platform.

"It is 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."

Anne Longfield, the Children's Commissioner for England, was shown the images uncovered by the BBC probe. "I have been very disturbed by what I have seen," she said, adding: "I would question whether humans are moderating this, are looking at this, and also I think it is failing to take account of the context of these images."

The probe came 12 months after the BBC found secretive groups being used by alleged paedophiles, including one being managed by a convict who was still on the sex offenders' register in the UK. Facebook claimed to have cleaned up its network since then.

But Damian Collins, chair of the UK commons media committee, said it has failed to police its own systems adequately.

"I find it very disturbing, I find that content unacceptable," he told the BBC. "I think it raises the question of how can users make effective complaints to Facebook about content that is disturbing, shouldn't be on the site, and have confidence that that will be acted upon," he added.

If you have concerns about a child's safety and wellbeing, including a child in your family, you can report it to the NSPCC online. Find out more here.