Britain's health secretary says the government will introduce new laws targeting online social media companies if they don't do more to protect children.

In a strongly worded letter to Facebook, Google, Snapchat, Twitter and others, Jeremy Hunt said their failure to prevent young children using social media and exposing children to its "harmful emotional side effects" was "unacceptable and irresponsible."

Hunt said he was particularly concerned about the lack of age verification measures, with thousands breaching minimum user age rules.

He gave the companies a week to set out steps they are taking to cut underage use, prevent cyberbullying, and promote limited screen time.

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Hunt last year attacked Facebook for releasing a version aimed at children, telling the company to "stay away from my kids."