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Apple has apologised following revelations that it paid third-party workers to listen to voice recordings of Siri users.

The practice known as "grading" has been used by several tech firms as a way of improving the quality of speech recognition.

However, Apple, Google and Microsoft all halted such work recently, following public outcry.

Apple said it plans to resume grading - but only for Siri users who opt in.

The firm added that in the future only its own employees will be able to access recordings, not third-party workers at contracted firms.

Earlier this month, the company said it had halted grading following reports that workers had heard recordings containing intimate remarks made by Siri users.

Such recordings can be made accidentally, for example when the Siri app interprets another word or noise as the utterance "Hey Siri", which is used to launch voice recognition.

Now, Apple says it has completed a review of such work.

"As a result of our review, we realise we haven't been fully living up to our high ideals, and for that we apologise," .

Up until grading was halted, 0.2% of Siri audio recordings was reviewed by human workers, Apple added.

The company said that three main changes would take place before human grading of Siri recordings was resumed. It said:

The turnaround was an unusual move from Apple, said Adam Wright, a tech analyst at market research firm IDC.

"I think they've been caught off-guard a little bit," he told the ´óÏó´«Ã½. "I don't think they've been completely forthcoming or transparent in their use of data."

Part of the controversy over using humans to grade voice recordings was that Siri users may not always have been aware that their conversations could be listened to in this way.

The Irish data protection authority, Apple's lead data privacy regulator in Europe, had previously said it was looking into the matter of grading.

A spokeswoman for the commission said it had noted Apple's latest statement.