Teaching, health and consumer groups have written to the government urging it to tighten up the rules on food adverts that target children.
The rules says ads should not encourage excessive consumption
They said a revised code for non-broadcast adverts, such as those on the internet, did not go far enough.
The National Union of Teachers and the British Heart Foundation are among 10 groups highlighting "inconsistencies" with the rules governing TV adverts.
The industry said there was no need for more restrictions like those on TV ads.
The new code for non-broadcast adverts, including those appearing on websites, was unveiled this month by the Committee of Advertising Practice (CAP).
Its measures - to come into force on 1 July - do not include "rules to restrict the volume or frequency of food advertising to children" and failed to "differentiate between healthy and unhealthy foods", said the 10 groups.
Their letter, sent to Health Minister Caroline Flint, said there were "major omissions and inconsistencies" between rules for print and internet adverts compared with those for broadcast adverts, which are developed separately by industry regulatory Ofcom.
The British Heart Foundation described the way food manufacturers target children on the internet as "unprincipled" and "unacceptable".
It said fast food firms like McDonald's and Burger King were using the internet and online games to target children.
McDonald's said it was a responsible advertiser and adhered to the code of practice, while Burger King said it was reviewing its communication with children, including on its website.
CAP's rules - applying to all foods except fruit and vegetables - say print and online media adverts for food and soft drinks should not encourage under-16s to be unhealthy.
Nor should they encourage excessive consumption, or use popular celebrities to target pre-school or primary-age children.
But the 10 groups said the advertising industry had done little more than repackage old rules, failing to put pressure on food manufacturers to come up with healthier products.
Which? food campaigner Miranda Watson said the CAP restrictions fell "well short of the mark".
And the consumer group's head of campaigns, Anna Butterworth, told BBC Five Live there were many loopholes - especially regarding the internet.
Food manufacturers and advertisers were using their own sites "to lure kids in to buy their unhealthy food products", she said.
She added: "There are a lot of loopholes and there's a lot of vagueness about exactly what restrictions are now in place, so it makes it very easy for food manufacturers and advertisers to continue doing what they're doing at the moment."
But Julian Hunt, of the Food and Drink Federation, which represents manufacturers, said it was "staggered" the new rules were being "attacked before they have been given a chance to work".
The Department of Health said it welcomed the new code but would prefer more "consistent" rules for adverts across all media.
It has promised to monitor the impact of the code and restrictions already imposed on broadcasters.
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 this month.
Restrictions are due to be extended to children aged up to 15 next year, along with an outright ban on promoting junk food on dedicated children's channels.
CAP said there was no need for similar restrictions on ads in non-broadcast media.
A committee spokesman said: "All of the limited evidence that exists for the effect of advertising of food to children relates to TV and not to other media."
He added it was appropriate for broadcasters to face tougher rules because of the greater influence of TV.
Other groups that signed the letter to Ms Flint were the National Heart Forum, National Consumer Council, National Children's Bureau, National Federation of Women's Institutes, Diabetes UK, Sustain and Netmums.