MPs took evidence from advertising executives, scientists and paediatricians in an inquiry on childhood obesity.
Items deemed high in fat, salt or sugar cannot be promoted on any media popular with children under advertising rules.
Former advertising executive and health campaigner Dan Parker said the rules fail to cover the most popular 50 websites.
He called for junk food marketing to be treated like pornography – with no screenings before the 9pm watershed.
Parker told the inquiry: “This code does not include Google or any other search engines. It does not include YouTube. It does not include Facebook or Twitter.
“If you take the list of the top 50 websites in the UK, not one of them is covered by this code.
“What it does is stop junk food advertising on websites likes Disney, Nickelodeon…who to the best of my knowledge they have never run this kind of advertising. It is a shocking sham, the whole piece of regulation,” he said.