Digital rights activists have pushed for law enforcers to examine BT's trials. Alexander Hanff submitted extensive documentation on Webwise to the City of London police, but the agency concluded no laws were broken.

Hanff people on Thursday to write letters to CPS in support of prosecuting BT for illegal interception.

Other U.K. government agencies have also examined Webwise. In January, the Home Office declared that targeted ad systems with user consent don't violate the law. The Information Commissioner's Office, which handles data privacy regulations, had said it will continue to monitor deployments of the system.

The ruckus over privacy issues lead BT to emphasize that users will be asked whether they want to participate. Webwise uses a cookie -- a piece of data stored in the browser -- in order to track a user's Internet activity.

The cookie contains an anonymous user ID, which is then associated with certain categories, such as "cameras" or "computers," which then determine what ads a person may see on Web pages that use Phorm to serve ads.