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Apple is facing a US$5 million lawsuit over a controversial feature of its latest operating system iOS 9.

The feature, dubbed “Wi-Fi assist,” automatically detects when your iPhone is using a weak Wi-Fi signal and forces it to use your cellular data instead.

The tool could be useful if you have a big data plan – but it’s turned on by default, which means those who don’t have big data plans may not notice when their phones have switched from using Wi-Fi.

Wi-Fi Assist sparked controversy shortly after iOS 9 was released in late September, after some users reported receiving hefty data overage charges thanks to the feature.

READ MORE: iOS 9 ‘Wi-Fi Assist’ feature could result in hefty data overage charges

Now, according to Apple news site AppleInsider, a California couple has launched a US$5 million class action lawsuit against Apple after receiving one of these hefty bills. The lawsuit was filed in a U.S. District Court in San Jose last Friday.

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The lawsuit alleges Apple did not give consumers enough information about how the feature works, leading the couple to incur overage charges on both of their iPhone 5S devices. The lawsuit did not disclose the total charges incurred by the plaintiffs.

Apple has not commented on the lawsuit.

Shortly after user complaints surfaced online, Apple updated its website to clarify that users “might use more cellular data” when the feature is turned on. However, the tech giant noted, “for most users, this should only be a small percentage higher than previous usage.”

Apple also clarified that Wi-Fi Assist only works when you have apps running in the foreground – it will not activate with apps running in the background of your device. It also won’t activate with certain third-party apps that stream audio or video – or download attachments like email apps – because they might use large amounts of data.

Here’s how to turn off Wi-Fi Assist on iOS 9:

Go to the “Settings” app on your iPhone and select “Cellular.” Scroll all the way to the bottom of the page and you will see the toggle for “Wi-Fi Assist.” Tap on the button to turn it off.