The Open Rights Group has asked seven major technology firms to opt out of BT's Phorm in order to protect web users' privacy.

Phorm uses browsing information to serve accurately targeted advertisements. Its creators claim that data collected will be anonymous, but the technology has been the subject of widespread criticism and scrutiny by parliament because of its implications for personal privacy.

An open letter from the Open Rights Group addressed to the chief privacy officers at major web site providers states: "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 system does not process any data that passes to or from your web site."

The companies concerned are Microsoft, Google, Facebook, AOL, Yahoo, Amazon and eBay.

The Open Rights Group has argued that Phorm is illegal because it allows web communications to be intercepted without the informed consent of the sender and receiver. The organisation has also maintained that the technology is unlawful because BT is making copies of copyrighted material without permission.

In the letter, the group points organisations towards the ability for firms to opt out of Phorm by sending an email to Webwise.

"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," added the letter.

Phorm is planned for a rollout in the near future under a Webwise brand by three UK internet service providers: BT, Virgin and TalkTalk.