Online advertising networks must allow web browsers to opt out of having their behaviour tracked, the Advertising Standards Authority has ruled.

Ad networks track browser behaviour by installing cookies on the user's machine. This allows them to build up a profile of the user in question and serve adverts that match their interests.

To date, ad networks only needed to gain permission to install cookies. From today, however, they must make it clear when they are tracking browser behaviour and provide some mechanism for browsers to switch it off.

The ASA does not dictate how the opt-out mechanism should work but recommends using an icon in the corner of ads that browsers can click to edit their preferences.

“The new rules will provide greater awareness of and control over [online behavioural adveritising], demystifying how advertisers deliver more relevant ads to us and allowing those of us who object to say “stop”," said ASA CEO Guy Parker. "We’ll be there to make sure that the ad networks stick to the rules.”

The ASA told Information Age that advertisers will follow the rules without threat of legal action, but added that there will be "sanctions available to us to make sure they are brought into line". "We are the established means for regulating advertising in the UK, so we have 50 years' experience of making sure advertisers comply with the rules," he said. "But we're not envisioning a problem with compliance and will monitor the situation for complaints and willingness for advertising networks to work with us."