A charity is complaining to the advertising watchdog about more than 50 websites it claims market junk food to children.
The Children's Food Campaign (CFC) said their "super-complaint" is against firms that promote unhealthy items, high in sugars, fat or salt, specifically to children.
A delegation from the organisation is taking its list of websites to the Advertising Standards Authority (ASA) offices in London.
CFC spokeswoman Kather Hashem said: "The pervasive nature of online junk food marketing to children really leaves us with no choice but to submit this 'super-complaint'. It is time for the ASA to face the music: will it or will it not act to protect children from cynical junk food marketing practices?"
The complaint follows a report published jointly by the CFC and the British Heart Foundation in December which highlighted the use of games, cartoons and social networking sites on some food and drink websites which it said targeted children.
The charity exchanged correspondence with Culture Minister Ed Vaizey following the publication of the report. Ms Hashem added: "This is not just our challenge, but comes from Minister Ed Vaizey, who has expressed his faith in the current self-regulatory system.
"When the picture is clear, we will inform the minister how the ASA has performed, so that he will be able to judge the effectiveness of the regulation for himself."
The CFC is calling for new advertising regulations to ensure unhealthy products are not promoted to children on the internet.
A spokesman for the Department of Culture, Media and Sport said: "As Ed Vaizey said in his letter, there are strict controls on the advertising of high fat, sugar, salt foods to children. Anyone concerned that a particular advert does not meet these controls should complain to the Advertising Standards Authority.
"The Government is taking action, including through Change4Life and the Responsibility Deal, to ensure children get the best start possible in life and to make it easier for families to make healthier choices and follow a balanced diet."