Controversial online advertising system Phorm the will be required to ask users to opt-in to stay legit, reports according to Information Commissioner's Office (ICO).
Phorm tracks web users' surfing habits so relevant online adverts can then be targeted at them, but it faces growing opposition from civil liberties groups and web experts. Tim Berners-Lee and the Foundation for Information Policy Research (FIPR) are among those that have slammed the system.
Read Is Phorm's targeted ad system illegal? here
"Phorm products will have to operate on an opt-in basis to use traffic data as part of the process of returning relevant targeted marketing to internet users," said the ICO in a statement.
"The Commissioner will keep the Phorm products under review as they are rolled out and his view will be strongly influenced by the experience of those users who choose to participate in any trials and the way in which they are able to make that decision," added the ICO.
See also: New online ad system 'spooky but safe'