Advertising technology company Phorm is to start a behavioural targeting trial in conjunction with BT today (Tuesday, September 30th).
Some 10,000 residential lines belonging to customers who have agreed to take part in the study will be monitored and profiled in real time.
Phorm's technology works by tracking an internet user's habits and then providing relevant advertising.
Technical problems and legal complications have delayed the start of the trial up until now - the study is the third conducted by the two firms.
According to Phorm: "BT customers are being invited to take part in the trial, which will take place over a number of weeks.
"Following successful completion of this trial and an appropriate period of analysis and planning, it is currently expected that Phorm's platform will be rolled out across BT's network."
Similar trials involving TalkTalk and Virgin Media subscribers are also thought to be in the pipeline.