Facebook must prove it is protecting children, says David Cameron In U-turn it said users should be able to watch and condemn these videos Hostage UK, which has Ken Bigley's brother as trustee, slams the move PM: Decision is 'irresponsible' and Facebook should 'address problem' Following pressure it has removed gory video of woman being beheaded Warning: Prime Minister David Cameron said Facebook had to prove it was protecting children David Cameron today challenged Facebook to prove that its system protected children after the site was forced to reinstate a ban on beheading videos. The Prime Minister led criticism against the social networking site after it lifted a ban on users posting graphic videos of violent killings. The firm yesterday bowed to pressure from child internet protection campaigners by taking down a graphic video of a woman being beheaded in Mexico. Facebook's unpublicised change in policy came to light after it refused to remove a 57-second video showing a young Mexican woman, her hands tied behind her back and on her knees, being beheaded by a masked man in broad daylight. It was posted last week under the title "Challenge: Anybody can watch this video?." Mr Cameron yesterday condemned Facebook's 'irresponsible' actions and demanded the firm 'explain their actions to worried parents. After the video was removed, the Prime Minister today welcomed the change of heart but warned youngsters using the site needed to be protected. Mr Cameron wrote on Twitter: "I'm pleased Facebook has changed its approach on beheading videos. The test is now to ensure their policy is robust in protecting children. Facebook had said that while the images must not be posted for "sadistic pleasure," they should be available for those who wish to condemn them. However, last night it was revealed Facebook had removed the clip of a woman being beheaded and said it would use a broader set of criteria to determine when gory videos are permitted on the site. Challenge: The Prime Minister used rival social media site Twitter to keep up pressure on Facebook The website had earlier begun adding alerts to videos showing decapitations or other extreme violence, saying: 'Warning! This video contains extremely graphic content and may be upsetting. "When we review content that is reported to us, we will take a more holistic look at the context surrounding a violent image or video," a Facebook spokesman said. Second, we will consider whether the person posting the content is sharing it responsibly, such as accompanying the video or image with a warning and sharing it with an age-appropriate audience. Terrible: The filmed executions of people like Ken Bigley would have been allowed to be shown on Facebook after they quietly lifted a ban - now reintroduced The change underscores a challenge for Facebook as it seeks to position itself as the go-to online destination where people share up-to-the-minute images and discuss breaking news events. The US firm acknowledged last night that its previous approach, which permitted the video of the woman's killing in Mexico to remain on its site, was flawed. Based on these enhanced standards, we have re-examined recent reports of graphic content and have concluded that this content improperly and irresponsibly glorifies violence. For this reason, we have removed it," the company said. Suicide prevention charities had denounced the original move to lift the ban on beheading videos, warning that the clips were "psychologically destructive" to teenagers. Others accused Facebook of double standards for allowing footage of beheadings but restricting what breastfeeding mothers can post and demanding women remove pictures of their mastectomy scars. After the video of the Mexcian woman being beheaded was posted, Facebook users reacted with outrage. One disgusted viewer commented: "Remove this video, too many young innocent minds out there shouldn't see this." Another wrote: "This is absolutely horrible, distasteful and needs to be removed... I'm very disturbed after seeing a couple of seconds of it. One of Facebook's advertisers, car-sharing companty ZipCar, withdrew its adverts in protest, saying: "We do not condone this type of abhorrent content being circulated on Facebook." Dr Arthur Cassidy, of suicide prevention group the Yellow Ribbon Program, said: "It only takes seconds of exposure to such graphic material to leave a permanent trace - particularly in a young person's mind." Another photo famously taken down showed Kylie Minogue holding a teddy bear with her microphone positioned in such a way, between the bear's legs, that made it look phallic.