David Cameron has attacked Facebook as "irresponsible" for lifting a ban on videos of beheadings being posted on its site.
The Prime Minister said the social network must explain its decision to allow images showing decapitations to "worried parents".
Facebook has said users should be free to view such videos and then condemn the content.
Mr Cameron wrote on Twitter: " It's irresponsible of Facebook to post beheading videos, especially without a warning. They must explain their actions to worried parents."
Facebook introduced a temporary ban on such videos in May but has since decided to remove the block on the grounds that the site is used to share information about world events, such as acts of terrorism and human rights abuses.
The company insists, however, that its approach would be different if the actions in the footage were "encouraged" or "celebrated".
Its new rules specify that videos and photos will be removed if they "glorify violence".
Stephen Balkam, a member of the Facebook safety advisory board, urged the company to rethink the change in its policy.
He told Sky News: "They have some very strict rules about nudity, about sex and even about violence too.
"I just think in this case they really need to rethink how they use and how they adopt their own policies."
A Facebook spokeswoman said: "Facebook has long been a place where people turn to share their experiences, particularly when they're connected to controversial events on the ground, such as human rights abuses, acts of terrorism and other violent events.
"People are sharing this video on Facebook to condemn it. If the video were being celebrated, or the actions in it encouraged, our approach would be different.
"However, since some people object to graphic video of this nature, we are working to give people additional control over the content they see. This may include warning them in advance that the image they are about to see contains graphic content."