Part of YouTube's system for reporting sexualised comments left on children's videos has not been functioning correctly for more than a year, say volunteer moderators.
They say there could be up to 100,000 predatory accounts leaving indecent comments on videos.
A BBC Trending investigation has discovered a flaw in a tool that enables the public to report abuse.
But YouTube says it reviews the "vast majority" of reports within 24 hours.
It says it has no technical problems in its reporting mechanism and that it takes child abuse extremely seriously. On Wednesday, the company announced new measures to protect children on the site.
YouTube is the world's largest video-sharing site. In addition to algorithms that can automatically block illegal and exploitative videos, it relies on users to report illegal behaviour or content that goes against its rules. The company says it has a zero-tolerance policy against any form of grooming or child endangerment.
Users can use an online form to report potentially predatory accounts, and they are then asked to include links to relevant videos and comments. The reports then go to moderators - YouTube employees who review the material and have the power to delete it.
However, sources told Trending that after members of the public submitted information on the form, the associated links might be missing from the report. YouTube employees could see that a particular account had been reported, but had no way of knowing which specific comments were being flagged.
BBC Trending was informed of the issue by members of YouTube's Trusted Flagger programme - a group that includes individuals, as well as some charities and law enforcement agencies. The programme began in 2012, and those involved have special tools to alert YouTube to potential violations.
The company says reports of violations by Trusted Flaggers are accurate more than 90% of the time. The volunteers are not paid by YouTube, but do receive some perks such as invitations to conferences.
With the help of a small group of Trusted Flaggers, Trending identified 28 comments directed at children that were clearly against the site's guidelines.
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The comments are shocking. Some of them are extremely sexually explicit. Others include the phone numbers of adults, or requests for videos to fulfil sexual fetishes. They were left on YouTube videos posted by young children and they are exactly the kind of material that should be immediately removed under YouTube's own rules - and in many cases reported to the authorities.
The children in the videos appeared to be younger than 13 years old, the minimum age for registering an account on YouTube. The videos themselves did not have sexual themes, but showed children emulating their favourite YouTube stars by, for instance, reviewing toys or showing their "outfit of the day".
The explicit comments on these videos were passed on to the company using its form to report child endangerment - the same form that is available to general users.
Over a period of several weeks, five of the comments were deleted, but no action was taken against the remaining 23 until Trending contacted the company and provided a full list. All of the predatory accounts were then deleted within 24 hours.
Source : BBC