Researchers say bans need to be part of wider strategy to tackle negative impact of mobile use on children.

Banning smartphones at school does not by itself improve academic grades and children's wellbeing, a study suggests.

Researchers from the University of Birmingham did find that spending longer on phones and social media in general was linked to lower grades, poor sleep, disruptive behaviour and a lack of exercise.

But these outcomes did not differ between schools that banned phones and those that did not.

The study, published in the Lancet's Regional Health Europe journal, also found that attempts to restrict phone use at school did not lower the overall time children spent on their devices throughout the day.

Instead, it said, any bans needed to be part of a wider strategy to lower phone use among students.

The peer-review study compared 1,227 students and 30 secondary schools.

It concluded: "There is no evidence to support that restrictive school phone policies, in their current forms, have a beneficial effect on adolescents' mental health and wellbeing or related outcomes."

But the research found increased screen time affected the mental health, classroom behaviour, physical activity and sleep cycles of students overall.

It said: "This study therefore provides further evidence of the adverse consequences from increased smartphone and social media use, and that lowering phone and social media use is important."

It added: "Our data suggest that interventions to reduce phone/social media time to positively influence adolescent mental wellbeing are plausible, but that both in-school and outside of school use should be considered in tandem."