Parents who leave the TV on for hours - even when no one is watching - can harm their children's speech development, say researchers.

Their study found that both adults and children use significantly fewer words when the TV is competing for their attention.

Researchers studied 329 children aged between two months and four years with the help of digital recorders that captured everything they heard or said.

Children heard an average of 770 fewer words from adults - or 7 per cent - per additional hour of television exposure, according to a report in the the Archives of Paediatrics and Adolescent Medicine.

Lead researcher Dimitri Christakis, professor of paediatrics at the University of Washington School of Medicine, said: 'We've known that television exposure during infancy is associated with language delays and attentional problems, but so far it has remained unclear why.

'This study is the first to demonstrate that when the television is on, there is reduced speech in the home.

'Infants vocalise less and their care givers also speak to them more infrequently.

'Adults typically utter approximately 941 words per hour. Our study found that adult words are almost completely eliminated when television is audible to the child.'

He added that every word counts in infancy and the findings suggest that exposure to TV or DVDs does not improve language skills.

'They could also explain why TV exposure is linked to other problems such as bad behaviour,' he said.

'Television actually reduces the number of language sounds and words babies hear, vocalise and therefore learn.

'We are increasingly technologising infancy, which may prove harmful to the next generation of adults'.

Previous research has found that DVDs and videos sold to help babies learn to talk actually slowed their language development.