A new iPhone setting could eat into your data allowance, as when you think you're using Wi-Fi it may automatically switch you to your phone's data. This could land you with huge bills or being cut off from using the web if you don't have an unlimited internet data package.

The problem affects the majority of people who upgrade to the new iOS 9 operating system, which is a free software update for iPhone 4s and later. So even if you don't have the new iPhone 6s or iPhone 6s Plus, you may still be hit.

Many millions of people have upgraded to the new software since it was made available on 16 September – Apple says more than half of users worldwide have. See our Cheap iPhones guide to get the best deal if you're thinking of getting a handset.

Warning! The new costly setting explained

The issue is very simple but possibly very costly.The new feature in question is called 'Wi-Fi Assist' and is available on all iPhones that support iOS 9, barring the iPhone 4s.

The Wi-Fi Assist settings clearly state it will "automatically use mobile data when Wi-Fi connectivity is poor". But if you have iOS 9 and do nothing, the default setting is that this function is switched on.

This will use your data allowance much quicker than you may expect, and in the extreme, could mean you have to pay for extra data if you end up above your limit and want to continue using more. Vodafone, for example, allows you to continue using it at £6.50 per 500MB.

Apple says Wi-Fi Assist isn't available if you're using data roaming, so you shouldn't be affected by this when using your phone abroad. That said, it's always best to turn off data roaming when abroad anyway to avoid racking up costly charges, and to instead use Wi-Fi. See our Data Roaming guide for more on this.

"I was stung on data usage"

While this is a relatively new problem, many iPhone users have already been caught out. Here are some of the complaints we've seen on Twitter.

How to avoid the data charge trap

If you're on a mobile tariff which doesn't include unlimited data, it's crucial that you check your settings as otherwise you could get a nasty surprise when your bill comes through, or face being cut off from using data.

The good news is that it's easy to stop this from happening. Head to Settings on your iPhone, then Mobile Data, then scroll to the bottom of the menu where you can turn Wi-Fi Assist off.

Does it affect iPads too?

Some iPads, which use data to access the internet as well as Wi-Fi, are also affected by this update – although it depends on the model you have.

The iOS 9 operating system is available for iPad 2 and later and iPad mini. All of these models feature Wi-Fi Assist barring the following: iPad 2, iPad 3rd generation, and iPad mini 1st generation. So if you have one of these three models, or an old version of the iPad you're not affected.

To turn off the Wi-Fi Assist setting, it's likely you'll need to follow similar instructions to those detailed above, although we've yet to receive confirmation from Apple of this.

Does it affect Android handsets too?

This shouldn't be an issue for Android users, as Google says Android devices do not automatically switch to using mobile data when Wi-Fi connectivity is poor. It says users have to actively opt in. Let us know if you've found this below.

While this article was originally published in September 2015, the information in it has been checked and is correct as of 8 Dec 2015.

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