The first stage of the restrictions, which came into effect on April 1, advertisements for HFSS (high fat, sugar and salt) foods - the term preferred by the Food Standards Agency to 'junk' - are not be permitted in or around programmes made for children (including pre-school children), or in or around programmes that are likely to be of particular appeal to children aged four to nine.
The measure was introduced as part of broader efforts to change children's eating habits and stem the rising tide of obesity.
However following a survey into children's viewing habits that showed many children do not just watch programmes aimed at their age-group, Which? is called for an advertising ban based around the 9pm watershed, rather than intended audience.
Such a watershed is already in place for programmes that contain violence or scenes of a sexual nature.
Which? looked at viewing figures released for terrestrial channels ITV1, Channel 4 and Channel 5 for two weeks in September.
The top five programmes for the under-10s were: Ant and Dec's Saturday Night Takeaway; The X-Factor; New You've Been Framed; Coronation Street; and Emmerdale.
Yet none of their programmes fall within the current HFSS food advertising restrictions.
Advertising of food products around programmes with the highest numbers of child views were said to include Mars Planets, Kinder Bueno, Milky Way, Smarties and Twix.
In fact, of the top ten programmes watched by under-10s, only two (The Simpsons and Spongebob Squarepants) were covered by the restrictions.
The Simpsons has a child audience of 163,200, Which? said, compared with 442,300 for the X-Factor.
Which? food campaigner Miranda Watson said: "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 instil healthy eating habits in their children."
The HFSS food advertising restrictions are due to be extended to children under the age of 16 years as of January 2008.
But Which? does not think this will make much difference to advertisement exposure amongst this group, since the September survey showed that only two of the top-20 programmes would be covered.
According to the UK's Department of Health, the prevalence of obesity (body mass index or BMI of 30 or above) among children aged 2 to 10 rose from 9.9 to 13.7% between 1995 and 2003.
The percentage of children aged 2 to 10 who were overweight (including those who were obese - that is, BMI of 25 or over) rose from 22.7% in 1995 to 27.7% in 2003.