Professor Yvonne Kelly, director of UCL's  International Centre for Lifecourse Studies, said the findings suggested a link between “the amount of time young people spend on social media and their levels of literacy”.

Both boys and girls who were heavier users were affected the same. “We looked at whether the more time young people spend on social media, the less time they have for the things that might improve their literacy such as reading for enjoyment and doing homework,” said Professor Kelly.

She said it was now time for the government to consider setting official “healthy” time limits on children’s social media use outside school. Her research also showed heavy users of social media – and particularly girls – were more likely to be depressed.