Verizon says it will soon offer customers a way to opt out from having their smartphone and tablet browsing tracked via a hidden un-killable tracking identifier.

The decision came after a ProPublica article revealed that an online advertiser, Turn, was exploiting the Verizon identifier to respawn tracking cookies that users had deleted.

Two days after the article appeared, Turn said it would suspend the practice of creating so-called "zombie cookies" that couldn't be deleted. But Verizon couldn't assure users that other companies might not also exploit the number - which was transmitted automatically to any website or app a user visited from a Verizon-enabled device - to build dossiers about people's behavior on their mobile devices.

Verizon subsequently updated its website to note Turn's decision and declared that it would "work with other partners to ensure that their use of [the undeletable tracking number] is consistent with the purposes we intended." Previously, its website had stated: "It is unlikely that sites and ad entities will attempt to build customer profiles."

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However, policing the hundreds of companies in the online tracking business was likely to be a difficult task for Verizon.

And so, on Monday, Verizon followed in the footsteps of AT&T, which had already declared in November that it would stop inserting the hidden undeletable number in its users' Web traffic.

In a statement emailed to reporters on Friday, Verizon said, "We have begun working to expand the opt-out to include the identifier referred to as the UIDH, and expect that to be available soon."

Previously, users who opted out from Verizon's program were told that information about their demographics and Web browsing behavior would no longer be shared with advertisers, but that the tracking number would still be attached to their traffic.

For more coverage, read ProPublica's previous reporting on Verizon's indestructible tracking and how one company used the tool to create zombie cookies.