The Open Rights Group has urged seven of the UK's biggest web companies to opt out of Phorm.
The group have sent an open letter to the firms - Microsoft, Google/YouTube, Facebook, AOL/Bebo, Yahoo, Amazon and eBay asking them to block Phorm, which tracks the sites a user visits to profile what adverts to display.
Jim Killock, executive director of the Open Rights Group, said: "Websites need to act to protect their user's data, and preserve trust in our basic privacy online. Phorm acts to gather data without the consent of website owners, and provides only an opt out system for them. That means all kinds of web content could be read without the consent of both parties."
The letter highlights the recent parliamentary meeting where Sir Tim Berners-Lee, inventor of the world wide web, made a "firm stand against technologies which 'snoop' on the internet" as well as drawing attention to a recent petition against Phorm's deployment that has over 21,000 signatures.
It also laid out the group's view that Phorm is illegal. "Communications cannot be lawfully intercepted, as this system does, without the informed consent of both the sender and receiver. The system will make copies of copyright material without permission, a further unlawful activity. Also, by forging extra tracking' cookies in your name, it may well bring your own system into disrepute."
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."
Killnock added: "We hope that major websites will send a strong signal that they wish to protect their user's rights, rather than allowing Phorm's snooping, which may be illegal."
