Many of the biggest companies in the world have pulled their advertising in response to the scandal. And YouTube has promised that it will remove any such comments or other content.

But they are proliferating across the site, and such videos are thought to have been watched millions of times.

The videos themselves are entirely innocent, of the kind that parents might shoot of their children and upload to be shared with friends. They might depict children taking part in gymnastics, for instance, or singing in front of a mirror.

But the comments posted under them are not innocent at all, as YouTube user Matt Watson showed in a video that brought the scandal to light. They often include suggestive remarks, and point out parts of the videos that might show children in compromising positions.

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Occasionally those videos might include explicit references and links to child pornography. Those known links were reported and have been removed.

What's more, they will include links out to other videos – in ways that will be spotted by anyone else who is also attempting to find such videos – and as people click, YouTube's algorithms will start to link them together so they will show up together in recommendations. Mr Watson claimed a "glitch" in YouTube's algorithm meant it was possible to find the videos "in about five clicks", and recommended content would quickly show dozens of similar clips of young girls, many of which featured similar comments.

It means that the comments themselves are not obviously breaking the rules, and might not even appear to be suspect if they were seen on their own. But taken together they represent a major operation that is enabling people to find videos in ways that could be harmful to the children involved.

That means that while YouTube can remove individual instances, and has promised to, the comments might continue to spread. It said it had taken an aggressive approach, disabling comments and terminating more than 400 channels.

A YouTube spokesperson said: "Any content – including comments – that endangers minors is abhorrent and we have clear policies prohibiting this on YouTube.