An extra hour a day of television, internet or computer game time between the age of 14 and 16 could reduce GCSE success by the equivalent of two grades, according to new research.
A Cambridge University study tracked the process of 845 students between the ages of 14 and 16 during 2005 to 2007.
It found those spending an extra hour a day on screens saw a fall in GCSE results equivalent to two grades overall.
Lead researcher Dr Kirsten Corder, from Cambridge University, said: "Television, computer games and internet use were all harmful to academic performance, but TV viewing was the most detrimental.
"We can cautiously infer that increased screen time may lead to poorer academic performance for GCSEs.
"I certainly wouldn't recommend banning television. But if teenagers or parents are concerned about GCSE results, one thing might be to look at the amount of TV viewing that they're doing and maybe just try to be sensible about it."
The research was part of a large study looking at different factors affecting the mental health, well-being and academic achievement of teenagers as they make the journey to adulthood.
Dr Corder added: "Even if you do sufficient homework, television viewing would still potentially lower your GCSE results."
The research also showed that levels of physical exercise and sport had no impact on GCSEs.