Having the TV on in the background reduces the quality and quantity of play in young children and may slow their development, according to researchers who studied children playing with and without background TV.

The researchers said it was a surprise that children were affected by an adult programme to which they appeared to be paying no attention. The long-term effect on a child's development is not yet clear, but the researchers advise parents not to have the TV on even if a young child is not watching it. "I would advise parents to be more cautious about background TV and turn it off when the child is in the room," said Dr Tiffany Pempek, a child psychologist at Georgetown University in Washington DC who worked on the study, reported in the journal Child Development.

"This is one of the first studies to look at background television," she added. "I think it can have a negative impact, but to say what the scope of that is yet, I think we need further research."

The team studied 50 children in total, aged 12, 24 or 36 months. The researchers watched the children playing in the same room as their parents for an hour. For half that time an adult gameshow was playing on the TV. Parents were asked to ignore the child unless it demanded their attention. The researchers compared the quality and quantity of play when the TV was on and when it was off.

The children appeared to pay little attention to the TV, glancing at it just 5% of the time, but the researchers found that when it was on the children's play episodes were on average 30 seconds shorter and they spent less time in focused play. These episodes, when the child is most attentive, were nearly 25% - about five seconds - shorter.

"We know from past research that focused attention, being able to focus like that, is an important predictor for [later development]," said Pempek.

The American Academy of Paediatrics, a professional body for paediatricians, recommends no exposure to TV and computer screens for children under two, but in the UK the Royal College of Paediatrics and Child Health has no official policy on the issue.