Controversial online advertising company Phorm said BT's plans to deploy its Webwise tracking system in the UK are taking longer than expected due to work to ensure the system can handle a large number of users.

Virgin and Carphone Warehouse are also trialling the system, which monitors the sites web surfers visit and keywords used in searches to deliver targeted advertising.

Phorm's service, Webwise, has come under fierce criticism from privacy activists who maintain that the system poses a data security risk. Phorm claims the system protects web surfers' anonymity while allowing ISPs to gain new revenue from online advertising.

BT's trial is designed to stress test Phorm to see if it can cope with large numbers of users, and to test their reactions. But despite the implementation of the system in the UK taking longer than expected, it's expected to be in place shortly.

"The main focus at the moment is the UK and getting to the trial and deployment stage with the three ISP partners here," said Justin Griffiths, who works for a public relations agency employed by Phorm.

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Phorm's progress in the UK contrasts with the bumpy road encountered by a similar company, NebuAd, which has worked to market its behavioural ad delivery system in the US.

The CEO of NebuAd, Robert Dykes, resigned on Wednesday and joined Verifone Holdings, an electronic payments hardware and software developer, as senior vice president. He remains listed as the founder and chairman of the company on its Web site.

Dykes testified before a US Senate committee in July, maintaining that NebuAd does not retain information that could later be used to identify individuals and their online activities.

NebuAd had deals with US ISPs to deploy its system, but some of those ISP partners have pulled out. On Wednesday, NebuAd said it has suspended deploying the system pending a Congressional inquiry, according to the Washington Post. NebuAd's London office could not immediately answer questions.

With additional reporting from Jeremy Kirk