The Open Rights Group has written to seven major web-based organisations asking them to opt out of the Phorm system.


The group has asked Google, Microsoft, eBay, Facebook, Amazon, Yahoo! and AOL, claiming it infringes on users' privacy that the owners can prevent the system looking at their sites.


In a letter, it said: “You may already be aware of the very significant concerns being expressed by many of your UK internet customers about the interception and processing of their data whenever it is viewed by customers whose ISPs deploy the Phorm/Webwise system.


“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. Making your decision public will provide reassurance to your customers, and will help them retain confidence in your brand, as well as in the integrity of the internet as a whole.”


The Webwise system from Phorm tracks people's web activity by using traffic information provided by internet service providers to help advertisers to target ads to people they think will be interested in them.


Privacy advocates have objected to trials of the technology they say were conducted without users' permission and say that the system does not inform users clearly enough of what it involves. The Open Rights Group and other activists have said that the system is only fair if users should actively choose to opt in or out of it.