Official complaints about internet ads rose by a third in 2006, according to new figures from the Advertising Standards Authority (ASA).
The agency reported 2,066 complaints - up from 1,577 and even more than direct mail attracted - according to the ASA's annual report.
"The internet is now the second most complained about non-broadcast advertising format - a rise unmatched by any other media," said ASA chairman Lord Borrie QC.
"Yet the boundaries of regulatory responsibility online are still unclear. The industry needs to address this issue quickly so that consumer faith in online messages can be as high as it is for advertising that appears in traditional formats."
Most complainants grumbled that the online ads were misleading.
Ads for gambling site Pool.com - which used the slogan "Why wait 'til 18? Bet at 16" - were pulled after a furore in which culture secretary Tessa Jowell called the campaign "utterly reprehensible" for encouraging young people to gamble, the report stated.
Complaints against email campaigns dropped by 10% to 276, however, and complaints against SMS adverts halved to 41.