Prime Minister David Cameron has waded into an escalating row about Facebook's decision to allow videos showing people being decapitated back on its pages.
He tweeted that it was "irresponsible" of the social network to post such videos without warning.
Facebook reversed a temporary ban on such videos, saying users should be free to watch and condemn such content.
The move was also condemned by one of the website's advertisers.
The row gained momentum when Mr Cameron tweeted: "It's irresponsible of Facebook to post beheading videos, especially without a warning. They must explain their actions to worried parents."
Car-sharing firm Zipcar told the BBC it was disappointed that its adverts had appeared next to one of the videos.
"We want you to know that we do not condone this type of abhorrent content being circulated on Facebook," it said in a statement.
"We have expressed to Facebook in the past the critical need to block offensive content from appearing and we will continue to engage with them on this important matter."
Facebook has since disabled Zipcar and other firms' ads from appearing on the page in question.
The BBC was alerted to the fact that such content was back online by a reader who said the firm was refusing to remove a page showing a clip of a masked man killing a woman, which is believed to have been filmed in Mexico.
It was posted last week under the title, Challenge: Anybody can watch this video?
The social network later confirmed it was allowing such material to be posted again.
"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," said a spokeswoman.
"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."
Following the Prime Minister's tweet she added: "Putting in place content guidelines that enable more than a billion people to express themselves while respecting the rights and feelings of others is a constant challenge.
"Our goal is to thoroughly examine our approach and take appropriate action, so that we can strike a delicate balance between enabling people to share information, news and content and protecting the community as a whole."
But the decision to end the ban has been criticised by Facebook's own advisors and law enforcers.
The chief executive of the Family Online Safety Institute, Stephen Balkam - who sits on the network's Safety Advisory Board - said he was "surprised" and "unhappy" at the move and would be contacting the firm to make clear his concerns.
After being contacted by a member of the local community, the South Australia Police (Sapol) force has also registered a complaint with Facebook about a video showing the decapitation of a woman.
Facebook advised Sapol it would not be removing the video.
Shelaye Boothey, director of Sapol's media and public engagement section, told the BBC: "Facebook advised us that they had reviewed the video and found, 'It did not violate our community standard on graphic violence'. Our team then spoke with a representative of Facebook about the decision, but were advised that the video would remain in place as a platform for community debate.
"Ultimately this is a decision that Facebook is entitled to make, however anyone concerned with the publishing of the video should continue to express their concerns through the appropriate Facebook channels."