Privacy activists have urged the internet's biggest technology websites to opt out of the controversial Phorm ad-targeting system, claiming it illegally intercepts web users' communications and could commit long-term damage to the brands who adopt it.

The Open Rights Group has written to Microsoft, Google/YouTube, Facebook, AOL/Bebo, Yahoo, Amazon and eBay, asking them to protect their users' privacy by opposing Phorm's Webwise service, which could be implemented by UK ISPs later this year.

Webwise tracks users' online surfing habits and then delivers relevant adverts - a practice that's raised a number of concerns from privacy campaigners. However, Phorm claims its 'anonymises' the information about web users so they are impossible to identify.

Following a successful trial of the service, BT looks set to be the first ISP to roll-out the system. It is thought it will be in place by the end of 2009, although BT has refused to commit to a deadline.

However, the Open Rights Group said a petition signed by 21,000 members of the public opposing the system demonstrated hostility to the technology from UK internet users.

"You may already be aware of our view that the Phorm/Webwise system is illegal. Communications cannot be lawfully intercepted, as this system does, without the informed consent of both the sender and receiver," the letter says.

The Open Rights Group also said web users had "very significant concerns" over the interception and processing of their data, and urged the those receiving the letter to opt out of Webwise by sending an email to website-exclusion at

"While we recognise that an 'opt-out' is an entirely second-rate way of dealing with this problem, we would strongly urge you to take advantage of it, in order to immediately reduce the risk of harm to your company and to your customers."

A Phorm spokesman said: "We are aware of the letter and note that the vast majority of recipients use or offer interest-based advertising. Many of them have, like Phorm, demonstrated their commitment to user privacy as signatories to the IAB UK's interest based advertising good practice principles."