BT (NYSE: BT) will rewrite part of its privacy policy as it rolls out one of the first implementations of a controversial new behavioural ad targeting system.

Virgin Media (NSDQ: VMED) and Carphone Warehouse’s TalkTalk have also signed up to sell data on customers’ web browsing habits to the Phorm network, which will use the data to serve up targeted ads on partner content sites when it launches later this year. BT is readying to trial the technology with 10,000 users, though each will be invited for their participation, The Register reports. To do so, the telco will have to amend the part of its privacy policy that reads: “We do not use this (customers’ browsing) information to analyse your visits to any other websites.”

Phorm is not without its controversy. Led by former Mig fighter jet tour operator Kent Ertugrul, it promises more relevant ads than even the farthest-reaching of web ad networks because it captures all user browsing behaviour, not just that generated from visitors to ad networks’ member sites. But as the clock ticks to Phorm’s launch, privacy advocates are sure to become more vocal.