Settings on the social network mean the groups are invisible to most users and only members can see the content.

Children`s Commissioner for England Anne Longfield said Facebook was not doing enough to police the groups and protect children.

Facebook`s head of public policy told the BBC he was committed to removing "content that shouldn`t be there".

A BBC investigation found a number of secret groups, created by and run for men with a sexual interest in children, including one being administered by a convicted paedophile who was still on the sex offenders` register.

The groups have names that give a clear indication of their content and contain pornographic and highly suggestive images, many purporting to be of children. They also have sexually explicit comments posted by users.

We found pages specialising in pictures of girls in school uniform - accompanied by obscene posts.

Images appeared to be stolen from newspapers, blogs and even clothing catalogues, while some were photographs taken secretly, and up close, in public places. One user had even posted a video of a children`s dance show.

Any Facebook user can set up a group - there are three settings options - open, closed and secret.

Secret groups cannot be found using the search facility and only members can see what content is inside them.

Only people invited by existing members can join the group.

The Internet Watch Foundation creates a list of web pages with child sexual abuse content and any found to be on Facebook groups are taken down automatically, Facebook says.

Any other material reported to the social network as inappropriate goes through an internal review procedure to check if it is in breach of its community standards.

Facebook says it removes content that includes "solicitation of sexual material, any sexual content involving minors, threats to share intimate images and offers of sexual services".

We set up our own fake profile and managed to gain access to some of these groups.

Using Facebook`s own reporting facility, we told the company about images and comments we thought were unacceptable.

In one secret group called "cute teen schoolies", we found a picture of a girl in a vest, aged 10 or 11, accompanied by the words "yum yum". Facebook responded that it did not breach "community standards" and the image stayed up.

In other secret groups we found pictures of children in highly sexualised poses. There were also innocent pictures stolen from other Facebook sites, school homepages and newspapers and most were accompanied with obscene posts. They also did not breach Facebook`s community standards.

We reported a whole group too - called "we love schoolgirlz" - with obscene content - and that did not get taken down either.

In total we reported 20 images. Users took some down themselves - Facebook removed four - leaving half still up.

We were so concerned about some material we found that we handed it over to the police. We also alerted both the Internet Watch Foundation and the National Crime Agency to the contents of our findings.

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