A random sample of BT customers will be asked if they want to join the trial
Telecoms giant BT is about to start further trials of a controversial internet advertising technology.
Developed by Phorm, the Webwise system watches what people do online and shows adverts tuned to their interests.
From 30 September, a sample of BT's customers will be invited to "opt in" to a trial of the technology.
Early trials ran without the consent of customers which led to complaints from rights groups who said this broke laws governing the interception of data.
The third trial was originally scheduled for March but has been repeatedly delayed.
Those that are invited to take part will see a special webpage appear when they start browsing the web. In a statement BT said customers would be able to opt in, opt out or ask for more information via the pop-up page.
A spokesman for BT said the trial would run for "at least" four weeks and that it hoped 10,000 customers would take part.
He said the technical trial would help BT assess whether the Phorm Webwise technology works well in the field.
Earlier trials of the technology suggested that BT would have to commit a lot of resources, potentially 300 servers, to use the system for all customers.
"If it goes according to plan it's our expectation that we will roll it out across the entire broadband customer base," said the spokesman.
No decision had yet been taken on whether Webwise would be "opt in" when the finished system is rolled out, he added.
The web browsing traffic of those that "opt out" will pass through the Webwise system but will not be profiled or copied by it, he added.
BT was also working on a separate system that let people opt out at a network level so their traffic avoided Webwise more completely, he said.
As well as serving up targeted adverts, BT said the Webwise system would also help to tackle fraud by warning people when they are about to visit a page that has been flagged as dangerous.
BT said it would make another announcement prior to starting the full roll-out of the service.