A Cambridge University researcher has called for BT to face prosecution for its "illegal" trials of Phorm, the controversial ad-serving technology.

Dr Richard Clayton, a leading computer security researcher, hit out at the telco after reading a leaked BT internal report.

Trials of the Phorm system in 2006 served up advertising that matched customer surfing details.

The monitoring system, which analyses users' surfing habits, has garnered interest from ISPs looking more accurately to target subscribers with advertising.

Phorm builds a user profile by sifting visited websites, and matching keywords with the content of the web page. Tailored advertising is then served up to users when they visit sites employing Phorm's technology.

"This is not how we expect ISPs to treat their customers' private communications," Clayton told the BBC.

"Since, not surprisingly, it is against the law of the land we must now expect to see a prosecution."

Phorm drew fire in March from the Foundation for Information Policy Research (FIPR) which claimed that the system was in contravention of the Regulation of Investigatory Powers (RIP) Act 2000 and was illegal in the UK.

FIPR argued in an open letter to Information Commissioner Richard Thomas that to ensure compliance with the RIP Act Phorm must not only secure the consent of web users but of website operators.

A BT spokesperson insisted that the company would press ahead with further trials later this summer.

"We have not announced a date yet. We are still planning. It will be quite soon," said the spokesperson.