The Mayor of London, Sadiq Khan, has singled out Google's YouTube for failing to crack down on online videos that "inflame" gang violence and knife crime.

YouTube has refused to take down four violent videos reported to them by the Metropolitan Police since December, which have been viewed 356,000 times. The Mayor's office says the videos depict gang members threatening rivals, describing how they would murder them, and making shooting gestures.

Khan said social media "can be used to inflame tensions and escalate violence quicker than ever before, and these videos are a shocking example of the glamorisation of gang culture".

He said current policies around violent content online do not go far enough. "Google, YouTube and other platforms have a responsibility to the millions of young people using their sites every day, and it is vital that they toughen up their guidelines, remove breaches immediately and work with partners to help ensure such horrific videos do not reappear. Lives could depend on it."

Claire Hubberstey, chief exec of Safer London – the charity that supports young people affected by violence – said these videos pose a serious threat to young people, "both glorifying violence and intimidation and posing a significant risk to those who appear in them".

She said: "We know that many of the participants are forced to appear in these videos and are often unaware of the grave danger they are in once they are posted. Such content contributes to young people feeling unsafe and increases the likelihood of them arming themselves as a result.

"It is essential that large corporations identify their role in safeguarding young people and commit to strict guidelines around their online content to help reduce this risk."

Earlier in the year, Google, Twitter and Facebook were accused in a select committee hearing of "having no shame" for not doing enough to combat the proliferation of online hate speech.

A YouTube spokesperson said: "While YouTube is a platform for free and creative expression, we strictly prohibit videos that are abusive or that promote violence and we have policy specialists that speak multiple languages based in countries and time zones around the world to review and remove flagged content that breaks our rules.

"We work closely with organisations like the Metropolitan police to understand local context and specifically, so that we can understand where artistic expression escalates into real threats. We're committed to continuing and improving our work on this issue and making YouTube a hostile space for those who seek to do harm." ®