The research reveals that more than half of the 20 programmes most watched by children under 10 are not covered by current rules.
Last month a report by the Advertising Association concluded that a pre-9pm watershed ban would be "a blunt instrument that would be disproportionate and unnecessary, given the new restrictions in place".
The current restrictions, which were introduced in April, ban ads for foods high in fat, salt or sugar from being shown in or around programmes aimed at, or which appeal to, the under 10s. However, the rules are based on the proportion of children watching a programme rather than the actual number.
This means that ads for unhealthy foods are not allowed during programmes such as 'The Simpsons' on Channel 4 and Sky One, which has a child audience of 163,200, but are allowed during programmes such as 'The X Factor' on ITV1, which has nearly three times the child audience at 442,300.
Which? found that 12 out of the 20 programmes most popular with under 10s were not covered by the current rules. It looked at the viewing figures for ITV1, Channel 4 and Five, for two weeks in September.
The research found that products including Mars Planets, Kinder Bueno, Milky Way, Smarties and Twix are being advertised during the programmes with the most child viewers.
In January 2008 the current rules will be extended to programmes with a high proportion of viewers under the age of 16, which would mean that only two of the top 20 programmes would be covered.
Mirand Watson, Which? food campaigner, 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 children."