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(Pocket-lint) - Please note, due to further investigation, this article has been updated. Please see the update towards the bottom of the story...

Pocket-lint has been approached by one of our readers who has shockingly discovered a potentially very damaging and expensive flaw in Apple's iOS. The flaw affects most of the company's 3G devices, including the iPhone 3GS, iPhone 4, iPhone 4S, the iPad and the iPad 2.

The dramatic problem relates to a setting in the iOS version of the App Store. In newer versions of the operating system - iOS 4 and iOS 5 - there is an option in Settings>Store that allows you to block downloads over a cellular network, thereby preventing a device from sucking down huge app, music or book files when not on Wi-Fi. And, by default, this is set to "off".

However, reader James Parmee Morris of Phone Support Worldwide, has informed us that this setting is broken, and it is entirely possible to download data over 3G from Apple's own store, without even knowing. It could also cost you plenty of cash in the process.

We have replicated the problem several times on different Apple devices and on different carriers in the UK, and can confirm that the Store cellular network setting is entirely useless when downloading apps under 20Mb. Above 20Mb a hard-coded setting will prevent downloads via 3G.

The problem also affects roaming users. Say, for example, that you allowed roaming on your device, for collecting emails and using Twitter, etc, but you wanted to prevent the phone from downloading apps over 3G. You would turn on the roaming, and normally expect the store setting to prohibit these updates and downloads. You would, of course, still be able to download using Wi-Fi.

But, according to our contact, if you then leave the Wi-Fi network, the phone will switch over to 3G, and continue to download the app updates, whilst incurring large data charges; T-Mobile would charge £150 for downloading 20Mb if you were in the US at the time, for example. Additionally, although the phone tries to warn you about this, downloads continue in the background and, therefore, incur fairly substantial download fees in so doing.

Also, it is possible to tell the Store settings to automatically download new purchases made on other devices, so if you download an app on your iPad, it's possible it will then automatically download to your iPhone over 3G. Even if you're not abroad, this constant data consumption may play havoc with your network contract - and incur extra charges along the way.

James tells us that he has informed Apple Customer Care about this issue, with the first call to them being made on 15 November 2011, but is yet to receive a single reply.

UPDATE: On further testing and investigation, it is our understanding that the "Use Cellular Data" option does indeed apply only to Automatic Downloads. However, while it is likely to be working as Apple intended, there are three things that are still troubling.

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Firstly, manually downloading apps from the App Store on Wi-Fi and then losing connection will still have the iDevice switch over to 3G to continue consuming data, without warning. You may receive a text message from your network after a certain amount of data has been downloaded, but by then it could still cost you a fair penny and be too late.

Secondly, should you switch on Automatic Downloads and "Use Cellular Data", forget (it could, after all, be a year or two later) and use Data Roaming abroad, you will still incur massive charges.

Finally, the way "Use Cellular Data" is given as an option can be misleading. Although it appears under a range of Automatic Download options, it is a reasonable assumption that there are many who would believe that it could be used generally for all downloads manual or automatic (as in this case). Therefore, should someone switch it on and then think it protects them from accidental downloads when on 3G only, they could be stung with a massive bill, whether there is an actual fault or not.

If nothing else, this should serve as an educational exercise for those who may not know their way around the intricacies of iOS as much as others.

It must also be pointed out that, while reader James has asked Apple Customer Care about this issue many times, they have never explained to him that it was for Automatic Downloads only. Surely, that would have been the simplest and quickest thing to do?

UPDATE 2: TechCrunch mobile editor Greg Kumparak (@Grg) has made a valid point via Twitter that partly confirms the above. The wording underneath the "Use Cellular Data" button ("Use Mobile Network Data" in British English) has recently been made much clearer for subscribers to iTunes Match: "That's not what that switch is for. "Flaw", no. Bad description, yes. Compare w/ iTunes Match on."

To illustrate, he also posted this example of two screenshots, side by side...

Pocket-lintRevealed The Apple iPhone flaw that could cost you thousands image 1

Have you had this, or any other, problem and been charged for downloads either in the UK or abroad? Let us know via the comments below...

Writing by Ian Morris. Originally published on 16 April 2013.