The Advertising Standards Agency is to extend its powers relating to online advertising.
At the moment, the UK watchdog can only look at paid-for ads such as banners and pop-ups.
But from 1 March 2011, its remit will be extended to include irresponsible and misleading advertising, and the protection of children, in all online marketing - including 'viral' marketing messages placed on social networks such as Facebook or Twitter.
The Committee of Advertising Practice, which creates the regulations governing the ASA's work, said it had decided to extend its powers in response to a formal request from a broad cross-section of UK industry.
ASA Chairman Lord Smith said: "We have received more than 4,500 complaints since 2008 about marketing communications on websites that we couldn't deal with, but from 1 March anyone who has a concern about a marketing communication online will be able to turn to the ASA."
In order to safeguard freedom of speech, the ASA will focus on ads that sell products rather than journalistic or editorial content.
The watchdog will be able to demand the removal of paid-for links to pages that host a banned advert, with the agreement of search engines.
The ASA will also be able to place its own ads online to highlight an advertiser's continued refusal to comply with a ruling.
Website owners and agencies have be urged to become familiar with the new rules before the March 1 deadline.
A document explaining the extension of the ASA's digital remit is available here.