AT&T says it has stopped using a controversial mobile technology that could be misused by advertising networks to track online users regardless of their wishes.

Until last week, the company had been inserting a unique identifier in web traffic sent by phones and other devices on its wireless network. It was doing this as part of a test program, which has now been stopped, says Emily Edmonds, an AT&T spokeswoman. News of this change was first reported by Pro Publica.

Privacy advocates hate these unique identifiers, which are also used by Verizon, because there's no way to turn them off. That means that they can be used by advertising networks to circumvent privacy tools such as do-not-track lists or private browsing settings.

As Google and Facebook make billions by selling data on their users, large internet service providers are looking at getting in on the action too. Verizon started its advertising program in late 2012. And while AT&T has now put its Relevant Advertising program on hold, the company says that it may return.

"We don't currently have a Relevant Advertising program in place, but we could have one in the future," says Edmonds. It could be this exact program that we tested, it could be something else entirely."