The Ulster Teachers' Union and the Northern Ireland Chest, Heart and Stroke Association (NICHS) say a revised code for non-broadcast adverts, such as those on the internet, do not go far enough.

The code was unveiled by the Committee of Advertising Practice (CAP) and will come into force on July 1 this year.

However, the UTU and NICHS fear the rules won't restrict the volume or frequency of food advertising to children or differentiate between healthy and unhealthy foods.

"These voluntary rules do not go far enough. We are concerned that junk food advertisers will use ways to evade the spirit of these guidelines," said Avril Hall Callaghan, general secretary of the Ulster Teachers' Union.

"If we are to reduce the obesity epidemic plaguing our children we need to do all we can to support them in making healthier choices, and not be unduly influenced by aggressive industry advertising."

Ms Hall Callaghan said that a growing number of Ulster schools are encouraging children to make proper food choices.

"We're seeing principals introducing healthy break times, for example, so all children are encouraged to bring a healthy snack.

"This means we don't have the situation any more where some have fruit and others have chocolate bars with the resultant peer pressure that can create.

"The feedback schools are getting from parents is extremely positive. Parents know how difficult it is for children to make healthy choices when faced with pressure from their friends, but when a school uniformly introduces a healthy break policy, it makes it easier for everyone - including mums and dads!

"Schools are also teaching children about the importance of a healthy diet and then, more importantly, backing up that message by offering children healthier choices at lunchtime too and we're hearing that children are increasingly aware of the healthy food groups."

However, the UTU leaders said that the message needs to be reinforced beyond the school gate.

"When children go home, they need to receive a message which is consistent with the good work being done between 9am and 3pm in schools - and teachers need the support, not only of parents, but also of the media."

Andrew Dougal, chief executive of NI Chest, Heart and Stroke, said he was disappointed not to see a total ban on the advertising of junk food to children.

"Food manufacturers and retailers are very clever. They can advertise a product as being low in fat, for example, but not mention that it contains large amounts of sugar or salt," he said.

"They also use viral marketing techniques, persuading children to pass on the message about a particular food to their friends. This is particularly prevalent on the internet.

"We owe it to our children not to condemn them to a life blighted by diabetes, heart disease and stroke because we failed as a society to counter slick marketing techniques."

Advertisements for foods high in fat, sugar and salt have been banned by Ofcom during or around TV programmes made for children, or programmes that would particularly appeal to seven to nine year olds, since the start of last month.