A number of global brands have pulled adverts from YouTube on the eve of one of the biggest shopping days of the year after they were alerted they were appearing with videos that have been exploited by paedophiles.
An investigation has estimated there are tens of thousands of predatory accounts, with paedophiles operating on the site and evading protection mechanisms.
In response, YouTube pledged to begin taking an "even more aggressive stance" against the behaviour.
Brands that have suspended ad campaigns on the site include Mars, Lidl, Deutsche Bank, Cadbury's and HP.
Investigations by BBC News and The Times found predatory accounts leave indecent comments on videos of children - some of the videos are posted by paedophiles and many are innocently posted by youngsters.
Some of the comments are said to be sexually explicit, while others reportedly encourage children posting the videos to perform sexual acts.
Anne Longfield, the Children's Commissioner, said the findings were "very worrying", while the National Crime Agency said it was "vital" online platforms have robust protection mechanisms in place when they are used by children.
The BBC and The Times spoke to people from the site's "trusted flagger" scheme who report inappropriate content or behaviour by users to YouTube employees.
Some of the volunteer moderators told the BBC there could be "between 50,000 to 100,000 active predatory accounts still on the platform".
As well as trusted flaggers, YouTube also uses algorithms to identify inappropriate sexual or predatory comments.
However, the system is said to be failing to tackle the problem and paedophiles are continuing to comment on videos of children.
According to The Times, adverts for several major international brands, including a global sportswear brand and food and drink giants, appear alongside the videos, raising concerns that they could be indirectly funding child abuse.
Several of the companies are reported to have pulled adverts from the site on the eve of Black Friday.
YouTube said it had noticed a growing trend around content "that attempts to pass as family-friendly, but is clearly not" in recent months and announced new ways it was "toughening our approach".
"Starting this week we will begin taking an even more aggressive stance by turning off all comments on videos of minors where we see these types of comments," said Johanna Wright, vice president of product management at YouTube, in a blog post.