Hong Kong (CNN Business) Facebook has admitted that it collected up to 1.5 million users' email contacts without their consent, in the latest privacy issue to hit the giant tech firm.

Facebook FB The world's biggest social network said Wednesday night that the email contact lists had been "unintentionally" uploaded tofollowing a design change almost two years ago, and the company was now in the process of deleting them.

Facebook said the issue began three years ago when it made changes to the step-by-step verification process users go through when signing up for an account on the platform. Prior to those changes, users were given the option to upload their email contact lists when opening an account to help them find friends already on Facebook.

But in May 2016, Facebook removed language that explained users' contact lists could be uploaded to the company's servers when they signed up for an account. This meant that in some cases people's email contact lists were uploaded to Facebook without their knowledge or consent.

A Facebook spokesperson said Wednesday the firm did not realize this was happening until April of this year, when it stopped offering email password verification as an option for people signing up to Facebook for the first time.

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