Unhealthy foods are still being advertised during programmes with high numbers of child viewers, despite government curbs, a consumer group study said on Monday.

Research conducted by Which? reveals that 12 out of the 20 most popular programmes with under-10s are not subject to the government's anti-junk food laws, and regularly feature advertisements for Mars Planets, Kinder Bueno, Milky Way, Smarties and Twix chocolates.

The programmes include The X-Factor, Ant and Dec's Saturday night Takeaway and New You've Been Framed.

Since April 1, advertisements for foods high in fat, salt or sugar have been banned in or around programmes made for children, or which are likely to appeal to children aged four to nine.

The laws are part of the government's attempt to address the UK's childhood obesity problem.

However, according to Which?, the laws are based on the proportion of children watching a programme, rather than on the specific number.

As a result, shows such as The X-factor are exempt from the curbs because although they attract thousands of young viewers, they also attract a large percentage of adult viewers.

Therefore, junk food advertisements are banned during The Simpsons, which has a child audience of 163,200, but are allowed during The X Factor, with a child audience at 412,800.

Which? is calling on the government to ban all TV commercials for food high in fats, salt or sugar before 9pm.

Which? food campaigner, Miranda Watson, said: "Tougher restrictions on the way unhealthy foods are marketed to children will play an important role in helping parents to instil healthy eating habits in their children."

However, director of communications at the Food and Drink Federation, Julian Hunt, says Watson's concerns are premature.

"The UK is now one of the most heavily regulated markets in Europe, and the new rules should be given time to take effect," he added.

Chief executive of The Advertising Association, Baroness Peta Buscombe, said: "Tackling obesity is about behavioural change. One has to question therefore why Which? are advocating a simplistic ban which is likely to produce unintended consequences and won't tackle the root cause of obesity."