Veronika Babochkina, 27, from Moscow, Russia, looks at the new iPhone 6s at The Apple Store in Opera, after queuing for few days, in Paris /

The tool lets people get around bad wireless connections — but it does so by connecting to the internet, which might not be so good if you’re on a limited data plan

One of iOS 9’s most useful features might be taking up all of your data allowance without you knowing it.

In the new update, released last week, Apple added a tool called Wi-Fi Assist. That switches away from slow wireless networks and instead uses network data, if that’s faster.

But in doing so it might mean the phone is using more data than you think. The feature switches seamlessly from a Wi-Fi connection to your network, so users aren’t necessarily aware that it’s happening.

The feature is on by default, but is easy to turn off.

Turning it off

Head to the Settings app and click Mobile Data or Cellular, near the top. Scroll down to the Wi-Fi Assist toggle and turn it off, when the switch should go white.

It’s then off, and can be turned back on in the same place.

How to check how much data you’ve used

If you want to keep the feature on but just want to be sure that you’re not using up all of your allowance, the iPhone offers the option of checking up on how much data it has used. In the same Mobile Data settings, you can see “Mobile data usage” for the current period, and you can choose to reset that every month.

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