Calls have been made to toughen up restrictions on which adverts can be shown during and around television programmes.
New rules introduced earlier this year banned adverts for foods high in fat, salt or sugar (HFSS) from being shown in and around programmes aimed at or which appeal to children aged under ten.
Consumer watchdog Which? says problems exist with this ban as it is based on the proportion of children watching a programme rather than the actual number.
This means that although adverts promoting unhealthy foods are not allowed during programmes such as The Simpsons they are allowed during other shows including X Factor.
Which? says that The Simpsons has a child audience of 163,200 while X Factor is viewed by 442,300 children.
The watchdog analysed television viewing figures for ITV1, Channel 4 and Five for two weeks during September to see which programmes children were watching in the largest numbers.
It claims that adverts during programmes with the most child viewers promoted products such as Mars Planets, Smarties and Kinder Bueno.
Current advertising rules will be extended to programmes with a high proportion of viewers under the age of 16 in January but Which? says if those rules applied today then just two of the top 20 programmes would be covered.
The group claims that its latest study provides more evidence of the need for a 21:00 GMT watershed for unhealthy food adverts.
Which? food campaigner Miranda Watson said: "This research shows that unhealthy foods are being advertised during the programmes with the highest number of child viewers, such as X Factor. The only way to shield children from TV ads for unhealthy foods is a 9pm watershed.
"There is no silver bullet in the fight against obesity but tougher restrictions on the way unhealthy foods are marketed to children will play an important role in helping parents to instill healthy eating habits in their children."