Watching television poses an even greater risk to children's health than previously thought, according to a report.

It claims an average child will have spent one year watching television by the age of six, and more than half of three-year-olds have their own set.

But its author, psychologist Aric Sigman, said the pastime has a massive impact on the future wellbeing of youngsters, with effects ranging from obesity to premature puberty.

Dr Sigman, a member of the Institute of Biology and associate fellow of the British Psychological Society, analysed 35 scientific studies and found a total of 15 negative effects.

He said that the risk applied to all programmes, even those publicised as "educational", and called on ministers to issue guidelines for parents.

He said: "Most of the controversy tends to be about children watching too much sex or violence. But, irrespective of what they are watching, there are still medical effects. Both the average number of hours watched and the age at which a child begins watching television are central to the association with negative effects later on.

"These salient issues occur at a time when the British population watches television for more hours and reads less than any other nation in Europe. To allow children to continue to watch this much screen media is an abdication of parental responsibility - truly hands-off parenting."

His report, published in the science journal Biologist, outlined 15 negative effects of watching TV from a young age.

They include an increased risk of becoming short-sighted and an increase in sleep disorders, body fat production and risk of cardiovascular disease.

Dr Sigman said exposure to TV in early childhood could also be a trigger for autism.

It can also impact later on in life as the risk of developing Alzheimer's disease increases with each extra daily hour of television among viewers between the ages of 20 and 60.

The research comes at a time when ministers are taking measures to tackle the health of children in Scotland, which has high rates of obesity, cardiovascular disease and type 2 diabetes.

Around one in five children in primary one in Scotland is already overweight, according to the latest official figures.

Dr Sigman called for guidelines on restricting the number of hours of TV and said toddlers under three should not watch any programmes.

He said: "People may think this is sensationalistic.

"There needs to be guidelines that we should not watch more than two hours a day and children under three should not watch any television."

A Scottish Executive spokeswoman said that it was tackling obesity through the National Physical Activity Strategy and the Scottish Diet Action Plan.

She added: "The Scottish Health Survey in 2003 found the time children spend sitting at a screen - other than at school - is a significant factor in not achieving recommended levels of physical activity of at least 60 minutes per day on most days of the week.

"Other studies have suggested that lower physical activity levels are strongly associated with levels of obesity among children.

"Obesity is not the consequence of a single factor and as such it is important not to look at the impact of physical activity in isolation from other factors such as diet.

"Behaviour change does not happen overnight and cannot happen through government action alone. However, we are confident our approach to tackling obesity is the right one, and this is further backed by the international endorsement we have received."