Just two hours of telly a day is leaving young children unprepared for school, a study shows.
And pre-school children are getting hooked on TV and computer screens in the same way as junkies are to heroin and cocaine, an education expert warned.
By the time they reach kindergarten children exposed to too much TV fare particularly poorly at maths, unable to control their emotions and have an undeveloped memory.
Scientists said too much TV is putting two to five year-olds back - with the affect felt most among those from poorer backgrounds.
Middle class children also suffered to a lesser extent, but there was no damage done to their more affluent peers.
The researchers said this could be down to well-off parents having more time to also watch the programmes which at the same time may be more educational.
The overall findings will alarm parents. Last year an Ofcom study showed pre-school children are spending more than four hours a day glued to screens.
They are online - using iPads and other devices - for an average of 71 minutes a day.
But - when gaming and watching television is included - their screen time rises to four hours and 11 minutes.
Commenting on the US study, Chris McGovern, of the Campaign for Real Education, said kids addiction to screens is not getting enough attention and blamed lazy parents.
He said: "It's easy to sit a child in front of a TV or tablet to keep them quiet. But it's not doing them any good.
"They become addicted very quickly and by the time they arrive in school this craving is fed even more.
"Schools have embraced digital technology much too much as well which is feeding this addiction."
He said: "It can do the same damage to children as older people taking cocaine or heroin.
"I'm not a technophobe. Heroin is good in small doses when given as morphine. But when you get too much you become an addict. That's what's happening to children with digital technology.
Mr McGovern said he has worked in schools in the South East where four and five year olds arrive and can't even speak properly.
He said: "That's because they've been sat in front of a TV from a very early age. That's the starting point. All they can manage is text speak."
The latest study of US families only looked at TV viewing and did not include video games, tablets and smartphone use.
It looked at data from 807 kindergartners of diverse backgrounds and found the number of hours of TV young children watch is related to decreases in their school readiness, mainly their maths skills and executive function.
The link was strongest when children watched more than two hours, reports the Journal of Developmental & Behavioral Pediatrics.
Study leader Andrew Ribner, a doctoral candidate in psychology at New York University, said: "Given studies have reported children often watch more than the recommended amount, and the current prevalence of technology such as smartphones and tablets, engaging in screen time may be more frequent now than ever before."
As family incomes decreased the problems increased, meaning children from low-income families are hurt more by too much TV.
Interestingly, TV viewing did not affect letter and word knowledge - possibly because programmes - especially those that are educational may work to improve literacy among young children in ways that are not found in maths.
The study did not measure the type of content the children watched - nor the context of their viewing.
But the researchers said both could be relevant - particularly in understanding why more affluent families appeared to be protected from the decline in school readiness linked to too much television.