BT is to start further trials of the controversial Phorm advertising system from 30 September.

Unlike the previous two trials, BT customers will be given the chance to opt-in to the system, meaning that they will not automatically be included in the trial. It has not been decided whether this will be the case with the final product.

This third trial was due to start in March but had been delayed due to legal problems.

Those invited to take part will be presented with a page that appears when they log-on to the internet. A message will say: "As a BT Total Broadband customer, you can try an exciting new service called BT Webwise."

BT hopes that the system will enable it to gain improved advertising revenues by targeting adverts more effectively.

The Information Commissioner's Office has ruled that the system does not breach the Data Protection Act as long as customers have a chance to opt out.

The European Commission has also examined the trials but has yet to take any action.

Phorm said in an announcement to the stock markets: "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."

It is not yet clear whether the final system will be opt in or opt out.