Jeremy Hunt has warned social media giants they face punitive action unless they stop children from seeing adult content on their sites.

In a powerful intervention, the Health Secretary has accused companies including Facebook and Instagram of 'turning a blind eye' to the harm being done to 'a whole generation of children'.

Firms could be hit with heavy fines unless they work out how to stop pre-teens from looking at age-inappropriate material.

Mr Hunt has been angered by the companies' failure to come up with effective child-protection strategies since he called in industry executives for a summit last November.

The Cabinet Minister, who has three children aged three, six and seven, has been alarmed by growing evidence that allowing children to access such sites on their phones, tablets or laptops is exposing them to cyber-bullying and damaging their mental health.

Health Secretary Jeremy Hunt has accused companies including Facebook and Instagram of 'turning a blind eye' to the harm being done to 'a whole generation of children' (pictured at the NHS confederation conference)

As part of the move, Mr Hunt has asked the Chief Medical Officer, Dame Sally Davies, to examine the evidence on links between underage internet use and depression and anxiety.

The move could lead to new Government advice on the amount of 'screen time' children should be exposed to – similar to the recommended number of weekly alcohol units and the 'five a day' advice on consuming fruit and vegetables. It is estimated an average three- to four-year-old spends three hours a day in front of a screen, rising to four hours for children aged five to 11 and more than six for teenagers.

When Mr Hunt met executives from Facebook, Twitter, Snapchat, Google and Microsoft last year he asked them to become 'part of the solution' to tackle growing levels of mental distress among children, which he said could pose as great a threat to children's health as smoking and obesity.

Now – infuriated by their lack of concrete proposals – he has urged them to come up with solutions on the problems of age verification, screen time limits and cyber-bullying by the end of the month.

In a letter to firms including Apple, Google and Twitter, Mr Hunt wrote: 'I fear you are collectively turning a blind eye to a whole generation of children being exposed to the harmful emotional side effects of social media prematurely.'

He told them it was 'morally wrong' and 'irresponsible for you to put parents in this position', and questioned whether an industry which 'boasts some of the brightest minds and biggest budgets globally' had 'sufficient will' to solve the problems.

Mr Hunt, who is increasingly talked about as a potential party leader, concluded the letter by threatening new legislation, saying: 'I am keen to work with you to make technology a force for good in protecting the next generation. However, if you prove unwilling to do so we will not be deterred from making progress.'

Most social media sites, including Facebook and Instagram, require users to be at least 13, but it is relatively easy to circumvent the checks.

The companies have complained to Mr Hunt that – short of requiring users to upload their passports – it is difficult to verify users' ages remotely.

Many countries have already acted. The US Department of Health recommends that children under two should not be in front of any screens, while in Taiwan parents who allow children under 18 to use screens for extended periods can be fined £1,000.