LONDON (Reuters) - Teenagers who spend an extra hour a day surfing the internet, watching TV or playing computer games risk performing two grades worse in exams than their peers who don't, according to research by British scientists.

In a study of more than 800 students aged 14 and 15, researchers from Cambridge University also found that physical activity had no effect on academic performance.

Since this was a prospective study, in which the researchers followed the pupils over time to see how different behaviors affected performance, the scientists said it was reasonable to conclude that too much screen time reduced academic achievement.

"We only measured this.. in Year 10, but this is likely to be a reliable snapshot of participants' usual behavior, so we can reasonably suggest that screen time may be damaging to a teenager's grades," said Kirsten Corder of Cambridge's Centre for Diet and Activity Research, who co-led the work.

The study, published in the International Journal of Behavioral Nutrition and Physical Activity, found the average amount of screen time per day was four hours.

An extra hour in front of the TV or online at age 14-and-a-half was linked with 9.3 fewer exam points at age 16 -- equivalent to two grades, for example from a B to a D. Two extra hours was linked to 18 fewer points.

Unsurprisingly, the results also showed that pupils doing an extra hour of daily homework and reading scored better - getting on average 23.1 more points than their peers.

The scientists said further research was needed to confirm the effect conclusively, but advised parents worried about their children's grades to consider limiting screen time.

In a breakdown analysis of different screen activities, the researchers found that TV came out as the most detrimental in terms of exam performance.

(By Kate Kelland)