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Chief cheerleader among the critics was Dr Richard Clayton, treasurer of the Foundation for Information Policy Research (FIPR) and a professor at Cambridge University, who was invited by the company to assess the technology underpinning its advertising scheme.

Speaking at the event, Clayton gave a damning inditement of Phorm, comparing it to Facebook's ill-fated Beacon advertising platform.

"Will serving up car adverts on a site about books be a good thing, or will it just annoy you? And if you have been looking for cars and you've finally bought one, you're really not interested in more adverts about cars. Of course if you've bought a car for your wife and you haven't told her... Facebook found out about that recently."

Clayton also said there were fundamental flaws in the way the system worked that could undermine the security strides made by browsers in recent years: "When you go browsing there's a certain amount of ping

"This is bad news - people are now starting to build system into browsers to spot this sort of thing because, currently, websites that behave like that can't be trusted, but suddenly Phorm is changing the world and all the websites are going to behave like that - it's really not helping to make the web more secure."

However, Phorm's senior vice president of technology, Marc Burgess, countered that this type of redirection would happen in fewer than 1% of browsing requests, and would affect neither speeds nor the experience.

Mission creep

The company also fended off concerns over "mission creep", the idea that once in place the technology could be used for data mining or further web surveillance at the behest of shareholders or unknown third parties.

"ISPs stand to lose far more in trust than anyone else. If anybody is not interested in mission creep, it's the ISPs," says Kent Ertugrul, Phorm's chief executive officer.

Ertugrul went on to suggest a panel of security experts be established to inspect the technology and audit its code periodically, without notice, to ensure it was adhering to its original function.

At the same event Ertugrul described the issue of "opt out" as a "huge red herring".