In a new report, the group claim that current measures to restrict television advertising for unhealthy foods towards children in the UK are not being effective enough.
Which? criticizes the restrictions in particular for failing to target the most popular television programs amongst young people.
According to its findings, child friendly shows like Spongebob Squarepants which attracted around 170,000 viewers between 4 -15 years of age in October - have been targeted by the ban, while shows like Ant and Decs Saturday Night Takeaway, watched by almost 1.5m people of the same demographic, were not affected.
In order to improve the efficiency of the measures, Which? claims that a post watershed ban (before 9 PM) - which would account for a number of primetime shows like Ant and Decs - is the best way to restricts advertisers influence on children.
"Which? Believes that a 9pm watershed is the only way to ensure that the restrictions are meaningful. If Ofcom cannot re-think its approach in the face of industry pressure, the Government needs to step in and legislate," said the groups chief policy advisor, Sue Davies.
The current restrictions on Junk-food advertising were announced last month by independent communications regulator Ofcom to prevent advertisers targeting under 16s with promotions for food and drinks high, in fats, salts and sugars..
On announcing the measures Ofcom stated that it aimed to protect children from junk food adverts "by the most targeted and proportionate means possible."
The regulator revealed therefore that it would target animated shows like Biker Mice from Mars, and Avatar: The Legend Of Aang, over prime time shows like Coronation Street and The X Factor.
Ofcom were unable to comment on the criticisms by Which?, but a spokesperson for the regulator revealed that consultations on the restrictions were ongoing until 28 December this year.
They added that Ofcom would wait until after this date before making any further announcements on the issue.