Young children who watch too much television have impaired language development, research suggests.
Some argue the very young should not watch TV
A review of studies on the subject for the National Literacy Trust says children aged two to five may benefit from good-quality educational TV.
The effect is enhanced when programmes are watched and discussed with an adult, said researcher Dr Robin Close.
But a lot of viewing, especially of programmes intended for adults, is associated with slower development.
She advises "strictly limiting the exposure of under-twos to television".
The chief inspector of England's schools, David Bell, has complained that some five year olds cannot speak properly when they start school.
The research review was published on Monday to coincide with a conference the trust has organised in London, called TV is Here to Stay.
Dr Close examined the findings of studies on English language development from a number of countries.
She found that, in the early years, there was evidence that attention and comprehension, language, knowledge of letter sounds and storytelling all benefited from high-quality educational programming.
Children who were heavy television viewers were less able to express themselves, though he says "specific cause and effect relationships have not been identified".
For those aged under two, the literature was far less certain about the benefits of "the current crop of educational television".
"Some evidence suggests that children under 22 months are simply incapable of acquiring information from television, or learning first words, and can only do so via interactions with adults," Dr Close reported.
Too much stimulation
Watching programmes aimed at a general or adult audience was associated with poor language development in pre-school children.
Researchers had found that the best TV involved content appropriate to children's ages, and which let them watch, interact and learn with adults.
Conversely, a negative experience went with excessive stimuli for the under-twos, complex narratives, watching with older siblings and watching adult programmes with adults.
"Where televisions are located in a child's bedroom, this is associated with reduced opportunities for co-viewing with parents and also with increased viewing of general or adult programming.
"The newness of the trend means that research has yet to fully explore the effects of children's viewing and behaviour patterns while watching television in their bedrooms."
A recent survey suggested that nearly a third of all children under the age of four have a television in their bedroom.
Dr Close said children watched more TV in poorer families where parents had little education.
Boys were more likely to watch than girls.
Children who spent less time at home, or watched with older children, especially in poorer homes, were less likely to see educational television.
"Carers should limit exposure for the under-twos in favour of other one-to-one language enhancing activities," she advised.
"Given the uncertainty within the literature, the cautious guidance of the American Academy of Pediatrics, which advises strictly limiting the exposure of under-twos to television, seems prudent."
The academy says it "does not recommend television for children age two or younger" and for older children, no more than one to two hours per day of educational, non-violent programmes.