Apple is facing a $5m (€4.5m) lawsuit after two users claimed the company didn't properly warn customers of a feature in the iOS9 update that could leave them with an empty wallet.

Wi-Fi Assist, which is part of the new update, is supposed to ensure users have the strongest Internet connection possible at every time of the day.

But it does this by automatically turning on Wi-Fi Assist if your mobile data or regular Wi-Fi is weak. 

Apple explain to function on their website:

"With Wi-Fi Assist, you can stay connected to the Internet even if you have a poor Wi-Fi connection. For example, if you're using Safari with a poor Wi-Fi connection and a webpage doesn't load, Wi-Fi Assist will activate and automatically switch to cellular so that the webpage continues to load."

William Scott Phillips and Suzanne Phillips, who filed the lawsuit claim that the result of Wi-Fi Assist is huge data charges for every user. They argue that Apple has failed to properly inform users of potential costs, and because of this, everyone should be reimbursed.

"Apple intentionally chose to have the default setting of the Wi-Fi Assist as activated while at the same time chose to omit the likelihood of data overcharges to consumers that do not have an unlimited data plan," Suzanne told AppleInsider.

While Apple say on their site that the increased data usage "should only be a small percentage higher than previous usage" the lawsuit argues this isn't the case.

"Reasonable and average consumers use their iPhones for streaming of music, videos, and running various applications — all of which can use significant data. Defendant's corrective statement does not disclose any basis for its conclusion that an average consumer would not see much increase in cellular usage."

The costly function is turned on by default so it might be worth checking your setting to make sure you don't find yourself with a hefty bill at the end of the month.