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The trial was linked to BT's deal with the advertising company Phorm, which uses details of users' browsing patterns to provide accurate targeted advertising.

Websites working with Phorm, currently including The Guardian and MySpace, will be able to place more relevant adverts using profiles built up from browsing histories.

A formal opt-in trial with the company is scheduled to be conducted this month with 10,000 BT customers, but a secret test already occurred in June of last year.

Affected users are threatening to take BT to court. "It is very likely that I and others will take legal action against BT for what they did last summer," said BT customer

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Stephen Mainwaring, speaking to The Register. Mainwaring noticed unusual DNS activity on his website in June last year, when BT claims it performed the test.

BT denies customer data was compromised. "BT can confirm that we conducted a very small scale technical test of a prototype advertising platform on one exchange in June 2007," says BT in a statement.

"The test was specifically conducted to evaluate the functional and technical performance of the platform. Absolutely no personally identifiable information was processed, stored or disclosed during this trial."

Online privacy is currently a hot topic, with BT, Virgin Media and TalkTalk all signing deals with Phorm to share their users' browsing details. The issue has even reached the attention of the web's creator, Tim Berners-Lee.

"I want to know if I look up a whole lot of books about some form of cancer that that's not going to get to my insurance company and I'm going to find my insurance premium is going to go up by 5% because they've figured I'm looking at those books," said Berners-Lee, speaking to the BBC.