Privacy campaigners have written an open letter to leading web companies demanding they resist the introduction of the Phorm web user tracking system by BT, Virgin and TalkTalk.
Phorm is a technology that can be used to track the web content viewed by web users to enable advertisers to send them targeted advertising.
BT and other firms have previously said the system will only be introduced to those subscribers accepting an opt-in to Phorm, but that has not stemmed the resistance of privacy campaigners.
The Open Rights Group, the Foundation for Information Policy Research, and prominent privacy advocates Alexander Hanff and Pete John, have written to Microsoft, Google/YouTube, Facebook, AOL/Bebo, Yahoo, Amazon, and eBay to win support for a boycott of Phorm.
The letter reads, "We are writing this open letter to you to ask you if you will act to protect your users' privacy.
"We are asking you to exercise your ability to opt out of the Phorm system, that is planned to be rolled out in the near future under the 'Webwise' brand by BT, Virgin and TalkTalk, the three largest UK ISPs."
The letter cites the opposition of world wide web inventor Sir Tim Berners-Lee to technologies which "snoop" on the internet.
It also says 21,000 members of the public have signed a petition against Phorm's deployment.
"We believe that many of your customers will feel exactly the same way. They may be using other internet providers, but the information they put on your website may well be viewed by them as personal, and they will not wish it to be read and stored by third-party technologies.
"Where your customers are using Phorm/Webwise ISPs, we are entirely unconvinced that the information they are given will ensure that they give 'informed consent' to the processing of all the data they send to and receive from your website."
The letter suggests those leading web companies may also have concerns of their own that a third party will be processing the contents of their websites, without asking their permission, to construct profiles of their customers.
The privacy campaigners claim that the Phorm/Webwise system is illegal, as they say communications cannot be lawfully intercepted, without the informed consent of both the sender and receiver.
They say the system will make copies of copyrighted material without permission, "a further unlawful activity".
"We strongly believe that it is clearly in your company's interest, it is in the interests of all of your customers, and it will serve to protect your brand's reputation, if you insist that the Phorm/Webwise system does not process any data that passes to or from your website," the letter adds.
The web companies are urged to use an opt-out process from Phorm published by BT.