YouTube says it will switch off comments on almost all videos featuring under-18s, in an attempt to "better protect children and families".
Several brands stopped advertising on YouTube after discovering that paedophiles were leaving predatory comments on videos of children.
YouTube had originally disabled comments on videos that were attracting predatory and obscene comments.
But it will now disable comments on almost all videos of minors by default.
It said the change would take effect over several months.
What was happening?
The BBC first reported that paedophiles were leaving explicit comments on YouTube videos back in 2017.
As well as leaving obscene or sexual comments, they were also using the comments section to signal content of interest to other paedophiles.
At the time, YouTube said it was "working urgently" to clean up the site.
However, in February this year advertisers including AT&T, Nestle and Hasbro suspended their ads after more predatory activity was found.
What has YouTube announced?
In a blog post, YouTube said its new policy meant videos of very young children would automatically have the comments section disabled.
The move is likely to include videos of toddlers uploaded by parents, as well as short films featuring children by established YouTube stars.
Videos of older children and teenagers will typically not have the comments disabled, unless a specific video is likely to attract predatory attention. That could include, for example, a video of a teenager doing gymnastics.
YouTube told the BBC it would use algorithms to detect which videos contained children.
Millions of hours of footage are uploaded to YouTube every day.
When will comments be permitted?
A small number of YouTube content creators will be allowed to enable comments on videos featuring children.
These channels will be trusted partners such as family video-bloggers or known YouTube stars.
However, they will be required to actively moderate their comments and demonstrate that their videos carry a low risk of attracting predatory comments.
YouTube said it had developed a system that was better at detecting predatory comments and removing them.
Previously, it had said it would stop video-makers earning ad revenue if paedophiles left explicit comments on their videos, but this will no longer be necessary.
What further action is being taken?
In addition to updating its comments policy, YouTube said it had terminated several channels that were "endangering" children.
The ban included several channels that were adding shocking content in the middle of children's cartoons.
It named FilthyFrankClips as one of the banned channels. It had released a video instructing children how to cut themselves.
"Nothing is more important to us than ensuring the safety of young people on the platform," said YouTube chief executive Susan Wojcicki on Twitter.
YouTube's app for children - YouTube Kids - has been criticised for using algorithms to curate content. Inappropriate videos have repeatedly been discovered on the service.
How have creators responded?
The comments left by fans on YouTube videos help the platform's algorithms decide which videos to serve up and recommend to viewers.
Creators have expressed concern that being forced to disable comments on their videos will affect the growth of their channels.
Despite the wide-ranging new policy, comments will remain part of the recommendation algorithm.
"We understand that comments are an important way creators build and connect with their audiences," YouTube said in a statement. "We also know that this is the right thing to do to protect the YouTube community."
Andy Burrows from the child protection charity NSPCC said the announcement was an "important step".
"We know that offenders are twisting YouTube videos for their own sexual gratification, using them to contact other predators and using the comments section as shop window to child abuse image sites," he said.
However, he called for an "independent statutory regulator" that could "force social networks to follow the rules or face tough consequences".