In the study, scientists found that the more television children watched aged two-and-a-half the worse they were at mathematics, the more junk food they ate, and the more they were bullied by other pupils.
As part of the research, study's lead author Dr Linda Pagani, of Montreal University, and her colleagues followed 1,314 children born in the Canadian state of Quebec in 1997 and 1998.
Parents were asked to report how much television their offspring watched aged 29 and 53 months, and teachers evaluated their academic, psycho-social and health habits when they reached the age of 10.
On average the two-year-olds watched 8.8 hours a week and the four-year-olds 14.8 hours.
Pagani and her research team found that for every extra hour of TV a week the two-year-olds watched there was a 6 per cent decrease in maths achievement, a 7 per cent decrease in classroom engagement, and a 10 per cent increase in 'victimisation' by peers, such as teasing, rejection and assault.
Each extra hour also corresponded with 9 per cent less exercise, consumption of 10 per cent more snacks, and a 5 per cent rise in body mass index.
Researchers said that pre-school is a critical time for brain development and that TV watching displaced time that could be spent engaging in 'developmentally enriching tasks'.
Pagani said that even incremental exposure to TV delayed development.
"Although we expected the impact of early TV viewing to disappear after seven-and-a-half years of childhood, the fact that negative outcomes remained is quite daunting," the Independent quoted her as saying.
The study has been published in the Archives of Pediatrics and Adolescent Medicine.