Children who are bombarded with junk food TV adverts will almost double the amount of unhealthy snacks and sweets they eat, alarming research has revealed.

Youngsters who are already overweight or obese are even more at risk and will increase their consumption by up to 134 per cent, the study shows.

The findings come from research at Liverpool University which draws a clear link between the TV advertising of foods high in fat, sugar and salt and rising obesity.

The revelation has triggered demands from health campaigners for a total ban on all TV advertising of junk foods up to the 9pm watershed.

And the findings will put pressure on parents to take greater control of both their children's TV viewing habits and diet.

Lead researcher, Emma Boyland, said: "These commercials go further than promoting brands, they are making children increase their total consumption of foods that are high in fat and sugar.

"Our research shows that overweight and obese children are most susceptible to food advertising. There is an exaggerated intake response.

"Given the obesity epidemic among children, there is a clear case that we should be protecting them more."

The study involved showing 59 youngsters aged 9 to 11 a total of ten typical food advertisements for popular snack foods around a Scooby Doo cartoon.

The commercials were for things like chocolate, sweets, savoury snacks, crisps, fast food, sugary drinks and breakfast cereals.

These youngsters were then given a selection of five foods by researchers who measured precisely how much was eaten by each child.

The results were compared with the food consumed by the same children who were shown ten toy commercials around a cartoon on another occasion.

Total food consumption by normal weight children rose by 84 per cent after seeing the snack food advertisements.

However, the amount rose by a higher 101 per cent for children who were overweight.

Obese children ate 134 per cent more after seeing the food advertising, compared to the TV session with cartoons and commercials for toys.

As part of the experiment, the children were offered Cadbury chocolate buttons, Haribo Starmix sweets, Walkers ready salted crisps, Quaker Snack-a-Jacks, and grapes.

These were similar to the products that were seen in the commercials but not the same brands.

Significantly, the children all ate much more of the foods that were high in fat, sugar and salt, than the healthier alternatives.

Consumption of chocolate buttons rose 150 per cent, crisps rose 93.5 per cent, while the children ate 91 per cent more of the Haribo jelly sweets.

The consumption of Snack-a-Jacks, which are a lower fat alternative to crisps rose by 27 per cent, while the children ate 41 per cent more grapes.

The advertisements used in the experiment were a snapshot of those shown around programmes aimed at children and families.

The researchers would not name the companies involved, however the biggest advertisers include McDonalds, Pepsi, Kellogg's Frosties, with its Tony the Tiger, and Coco-Pops, KFC and Pepsi, who has used David Beckham.

Importantly, the increase in the intake of food was not directly related to the brand that was featured in the advertising.

Of the 59 children in the survey, 33 were normal weight, 15 were overweight and a further 11 were obese. Some 32 were boys and 27 were girls.

The TV watchdog, Ofcom, has announced some new controls on the advertising of unhealthy foods.

From the beginning of April, advertisements for junk food were banned around programmes made specifically for children aged up to nine.

This will be extended to programmes where children aged up to 15 make up a high proportion of the audience from January next year.

Dedicated children's channels will have to remove all advertisements for unhealthy foods by December of 2008.

Critics say the system is flawed and will not prevent firms from advertising unhealthy foods around programmes watched by most under 16s, such as X-Factor or Coronation Street.

Miss Boyland said: "I don't think Ofcom has gone far enough. The restrictions only really cover programmes specifically aimed at children, not the programmes that are watched by more children than any others.

"It needs to think about extending controls up to the 9pm watershed."

Dr Jason Halford, director of the Liverpool University's Kissileff Laboratory for the study of Human Ingestive Behaviour, said: "Our research confirms food TV advertising has a profound effect on all children's eating habits - doubling their consumption rate."

The university research team is presenting its findings at the European Congress on Obesity in Budapest this week.

A host of health and consumer groups, ranging from the National Heart Forum, to the BMA, Which? and the WI have backed a TV advertising ban through to 9pm.

The Advertising Association, which speaks for agencies making commercials, rejected the Liverpool research.

Spokesman, Jonathan Collett, said: "Ofcom's extensive research showed that TV advertising has only a modest direct effect on children's food preferences and that obesity is caused by other more significant factors such as exercise and lifestyle choices."