BT conducted two "small scale" trials of the Phorm system
BT should face prosecution for its "illegal" trials of a controversial ad-serving technology, a leading computer security researcher has said.
Dr Richard Clayton at the University of Cambridge made his comments after reviewing a leaked BT internal report.
The document reveals details of a 2006 BT trial with the Phorm system, which matches adverts to users' web habits.
"It's against the law of the land," he told BBC News. "We must now expect to see a prosecution."
But BT plans to push ahead with a further trial of the technology later this summer, the BBC has learnt.
"We have not announced a date yet; we are still planning - it will be quite soon," a spokesperson said.
Revelations about earlier trials have prompted some customers to organise protests in London to coincide with BT's AGM on 16 July.
The company did not inform customers that they were part of the original tests in 2006 and 2007, although 30,000 subscribers were involved.
Nearly 19 million web pages were intercepted during the 2006 tests, according to the leaked report, posted to the whistle-blowing website Wikileaks.
BT described it as "a small-scale technical test".
During the trials adverts were stripped out of web pages served up to BT customers and replaced with more targeted ads, if available.
If none was available, adverts for one of three charities were inserted.
The report also reveals that BT believes that a large scale deployment of Phorm will be "operationally challenging".
It projected that it would require as many as 300 servers to roll out for all BT customers and added that BT was "unable to find hosting accommodation that will enable a deployment of this model".
However, the BBC now understands that the technology behind Phorm has been updated and large-scale trials are much more feasible.
Dr Clayton said the leaked report "clearly shows that back in 2006 BT illegally intercepted their customers' web traffic, and illegally processed their personal data".
He continued: "The BT author seems delighted that only 15-20 people noticed this was happening and looks forward to a new system that will be completely invisible.
"This isn't how we expect ISPs to treat their customers' private communications and since, not surprisingly, it's against the law of the land, we must now expect to see a prosecution."
He said that the BT report also noted that "communications regarding advertisement systems and information collection could lead to negative perception if not carefully handled".
"They seem to have failed in this aspect as well," he said.
A BT spokesman said: "The trial was completely anonymous and no personal information was stored or processed.
"BT sought expert legal advice before commencing the trial."