An error has led to a PlusNet broadband customer being sent an invitation to take part in a trial with BT.
Phorm has an agreement to test its advertising targeting technology with BT customers that volunteer to take part in the project.
Under the system, participants are targeted by marketing campaigns according to the nature of their browsing activity.
PlusNet said in a statement: "PlusNet customers using RIB should not normally see the BT Webwise invitation screen or be able to join the BT Webwise trial.
"This case is believed to have been due to a configuration error, which we acted on as soon as we were made aware of it."
PlusNet is a subsidiary of BT but is run as a separate entity from the telecoms giant.