A report released by the group said that companies would still be able to advertise foods with HFSS content in ad breaks during programmes popular with a young and older audience, including ITV programmes 'The X-Factor' and 'Ant and Dec's Saturday Night Takeaway', which are regularly watched by more than 1m under-16s.
Ofcom said in November it would be introducing a ban on the advertising of junk food during children's TV programmes at the end of March 2007.
Which? has based its results on a two-week snapshot of TV viewing figures for ITV1 in October, which found that many children aged four to 16 watched ads in the early evening rather than first thing in the morning.
Among the programmes likely to be unaffected by the ban according to Which? are 'Who Wants To Be A Millionaire?', 'Coronation Street', 'Ant & Dec's Saturday Night Takeaway', which attract up to 1m under-16s. However, shows on channels like Nick Jr, such as animated series 'Spongebob Squarepants', which pulls around 170,000 viewers under the age of 16, would be affected.
Although Ofcom's junk food ban is expected to cost the industry around £39m in TV ad revenues, health campaigners have said the measures fall short of the total ban of junk food commercials before 9pm that many health practitioners called for.
Sue Davies, chief policy adviser at Which?, said: "While Ofcom has recognised that its objective should be to protect children under 16, its proposed approach is completely flawed. Producers of high in fat, sugar and salt will still be free to advertise their products during the programmes most children are watching.
"Which? believes that a 9pm watershed is the only way to ensure that the restrictions are meaningful. If Ofcom cannot rethink its approach in the face of industry pressure, the government needs to step in and legislate."
A spokesperson for Ofcom rebuffed Which?'s findings, stating: "For every child watching the programmes listed there is typically nine adults. It would not be targeted or proportionate to impose a blanket ban before 9pm."
Ofcom said it was consulting on the junk food ad ban measures until December 28, and would make a further announcement in early 2007.
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