UK regulations were introduced five years ago banning the advertising of foods high in fat, salt or sugar during any children's programming on television.
However, researchers from the University of Newcastle said that the number of junk food ads seen by children actually increased after the ban, from 6.1% to 7%.
The academics studied the "nutritional content" of television adverts six months before the new restrictions were introduced in 2007, and then again six months after the full restrictions were put in place in July 2009.
After linking the data to how many people saw the adverts, the researchers discovered that 14.6% of commercials were for food and half of those were for less healthy items such as crisps, sugared breakfast cereals and drinks containing large amounts of sugar.
Although nearly all the ads shown during children's programmes adhered to the rules, the team found that children were not only watching programmes intended for them.
Dr Jean Adams, a lecturer in Public Health at Newcastle University, said that advertisers are staying "within the letter of the law" but are "not getting the spirit of the law".
"These regulations were brought in to help young people make better lifestyle choices and encourage a healthier diet," said Dr Adams.
"However, what they are seeing is exactly the same amount of advertising for food which is high in salt or high in sugar and fat as before the regulations came in.
"We know advertising works - otherwise food companies wouldn't use it - so we have a duty to further tighten up the restrictions particularly if we're going to help our young people grow up to make good choices about the food they eat."
Media regulator Ofcom brought in the restrictions on junk food advertising around programming aimed at children as part of efforts to tackle rising levels of childhood obesity.
In 2010/11, 11% of boys and 9% of girls aged four to five years were obese; while 20% of boys and 17% of girls in aged 10 to 11 were obese. By 2050 obesity is predicted to affect 60% of adult men, 50% of adult women and 25% of children.
The Newcastle University researchers found that the proportion of food advertising on TV featuring products deemed unhealthy actually increased after the Ofcom restrictions, from 38.6% to 60.4%.
But Ofcom has doubted the findings, claiming that its research shows that there was actually a decrease in the number of unhealthy food ads watched by young people.
An Ofcom spokeswoman said: "We note the research from the University of Newcastle. Our final review of the rules, which included a full year of data from 2009, showed a significant reduction (37%) in children's exposure to adverts for products that were high in fat, salt and sugar since 2005."
Reacting to the report, a Department of Health spokeswoman said: "Being overweight and not eating well is bad for our health. Controlling the advertising of food to children is important, but it is only part of the picture.
"We are taking action, including through Change4Life and the Responsibility Deal. We want to make sure that children get the best start possible in life and to make it easier for families to make good choices about food.
"Ofcom introduced significantly tougher restrictions on advertising foods high in fat, salt and sugar to children in 2007."
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