Psychologist Dr Aric Sigman says:

Children of all ages are watching more screen media than ever, and starting earlier.

The average 10-year-old has access to five different screens at home, he says.

And some are becoming addicted to them or depressed as a result, he warns.

Writing in the Archives of Disease in Childhood, Dr Sigman says a child born today will have spent a full year glued to screens by the time they reach the age of seven.

He adds: "In addition to the main family television, for example, many very young children have their own bedroom TV along with portable hand-held computer game consoles (eg, Nintendo, Playstation, Xbox), smartphone with games, internet and video, a family computer and a laptop and/or a tablet computer (eg iPad).

"Children routinely engage in two or more forms of screen viewing at the same time, such as TV and laptop."

'Facebook depression'

British teenagers are clocking up six hours of screen time a day, but research suggests the negative impacts start after two hours' viewing time.

Dr Sigman cites from a string of published studies suggesting links between prolonged screen time and conditions such as heart disease, stroke and diabetes.

But he suggests the effects go further than those simply associated with being sedentary for long periods.

He says prolonged screen time can lead to reductions in attention span because of its effects on the brain chemical dopamine.

Dopamine is produced in response to "screen novelty", says Dr Sigman.

It is a key component of the brain's reward system and implicated in addictive behaviour and the inability to pay attention.

"Screen 'addiction' is increasingly being used by physicians to describe the growing number of children engaging in screen activities in a dependent manner," Dr Sigman says.

'Reduce screen time'

And there are other psychosocial problems associated with excess screen time. These include "Facebook depression", reported by the American Academy of Pediatrics, which develops when young people spend too much time on social media sites and then begin to exhibit classic symptoms of depression.

Dr Sigman says: "Perhaps because screen time is not a dangerous substance or a visibly risky activity, it has eluded the scrutiny that other health issues attract."

He says there are many questions remaining about the precise nature of the association between screen time and adverse outcomes, but adds: "The advice from a growing number of both researchers and medical associations and government departments elsewhere is becoming unequivocal - reduce screen time."

Developmental psychopathology expert Prof Lynne Murray, of the University of Reading, said: "There is a well-established literature showing the adverse effects of screen experience on the cognitive development of children under three, and the US Paediatric Association for example has recommended no screen time before this age.

"If children do watch, however, adverse effects are mitigated by watching with a supportive partner - usually adult , who can scaffold and support the child's experience, and by watching more familiar material.

"A lot of screen material is not well designed for a child's cognitive processes, eg loud, fast changing stimulation - this is attention grabbing, but does not help processing."