Children will be free to watch explicit X-rated sex videos on social media sites because of a loophole in a new porn crackdown, Britain’s chief censor has admitted.

The government is also braced for criticism next week over an anticipated delay in its prospective curbs on under 18s’ access to hardcore porn sites.

David Austin, chief executive of the British Board of Film Classification (BBFC), has been charged by ministers with enforcing new laws that require people to prove they are over 18 to access porn sites.

However, writing for telegraph.co.uk, Mr Austin admitted it would not be “a silver bullet” as online porn on sites such as Facebook and YouTube would escape the age restrictions.

“Social media companies will not be required to carry age-verification for pornographic content on their platforms,” he said.

He said it was a “matter for government to review this position” which comes despite evidence from the London School of Economics (LSE) that children under 16 view most of their porn on social networking sites or in films and TV. Some 40% of boys aged 14 to 17 admit regularly watching porn.