The ad-serving system profiles the sites people visit online
Seven of the UK's biggest web firms have been urged to opt out of a controversial ad-serving system.
Phorm - aka Webwise - profiles users' browsing habits and serves up adverts based on which sites they visit.
In an open letter, the Open Rights Group (ORG) has asked the firms to block Phorm's attempts to profile their sites, to thwart the profiling system.
Before now, Phorm has defended its technology saying that it does not break data interception laws.
Chief privacy officers at Microsoft, Google/Youtube, Facebook, AOL/Bebo, Yahoo, Amazon and Ebay have been sent copies of the letter signed the digital rights campaign group and anti-phorm campaigners.
In it, the seven firms are urged to protect their users' privacy by refusing to work with Phorm.
It says many members of the public have "very significant concerns" about the way that Phorm collects and processes data about their web-browsing habits.
The campaign group pointed out that more than 21,000 people had signed a petition asking for Phorm to be investigated and banned if it was found to break European laws on personal privacy.
The Phorm system works by looking at the web traffic generated by a person as they move around the web, and then serving up adverts based on their browsing history.
While people can opt out of Phorm, their web traffic will still be collected though not profiled by its ad-serving technology.
In the UK, BT, Virgin Media and TalkTalk have expressed an interest in using Phorm. BT has conducted several trials and has declared its intent to deploy the system.
Jim Killock, director of ORG, said it sent the letter to highlight that it was not just webpages prepared by commercial organisations that would be analysed and profiled by Phorm.
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Information that people put on social networking sites, in blogs or on their own websites would be used by Phorm to profile someone else's browsing habits.
"Because it's a webpage, it can be read and used in that system," said Mr Killock.
He said UK laws on intercepting data called for consent from the owner of the data to have it used in this way.
"We cannot really see that they have had consent off people to use it," he said.
Phorm was given a qualified legal all-clear by the Information Commissioner in early 2008. He ruled that the firm had not breached guidelines on the use of personal data or the methods it used to monitor those enrolled in the system.
However, the Commissioner said Phorm would have to get permission from users if the data collected was used for "value added services".
A spokeswoman for Phorm said it was aware of the ORG letter, and added that most of the firms it had been sent to were already using systems similar to the interest-based advertising it offered.
"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," she added.