The UK's Office of Fair Trading is currently investigating whether free-to-play (aka freemium) games are unlawful, in consideration of the recent outcry over in-app purchases being available to kids.

Some parents have found that their children have racked up massive bills by buying add-ons and extras in games, and the OFT is contacting companies which develop such titles to determine whether their in-app purchasing platforms are "misleading, commercially aggressive or otherwise unfair".

Real Racing 3 is perhaps the flag bearer for free-to-play games, with many bemoaning its in-app costs, saying that it is nigh-on impossible to progress without spending bundles of real-world cash (although Pocket-lint has found that it can get plenty of hours' playtime without coughing up a single penny). But there are many other examples, such as Candy Crush Saga, which requires a payment to unlock the next set of levels, or Angry Birds Star Wars, which also has new sections hidden behind paywalls.

Until the OFT makes its decision and announcement, you can actually do something yourself to stop hefty bills being run up in your name. You can stop in-app purchases either altogether or hide them behind a PIN, so that your child needs your permission to proceed. And here's how.

How to turn off in-app purchases on iPhone, iPod touch or iPad

On an iPhone, iPod touch or iPad, hit Settings to bring up the menu. Scroll down to General and tap that option.

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In General you will see a sub-heading called Restrictions. By default it will be switched off, so hit that and you will go to a sub-menu listing all the apps and modes you can restrict.

At the top of this page, there will be a strip that says "Enable Restrictions". Tap on that and you will be asked to enter a PIN code. Choose one that you can easily remember and enter it twice - the second time to confirm.

You will then see that this menu has opened up to you. Scroll down to In-App Purchases, which should be switched to On. Hit that slider and you have successfully turned off the ability to buy more content within games (and any other apps). Exit.

To turn on the mode on again you will need to navigate back to the Restrictions menu, re-enter your passcode and slide the slider back.

How to turn off in-app purchases in Android

Android is different from iOS in that you can hide in-app purchasing behind a PIN code, but not switch it off entirely.

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To do so, open the Google Play store app, and enter the Menu. Tap on Settings in the Menu. On the next screen, scroll down to Set or change PIN and hit that option.

A pop-up box will appear that will allow you to set your own PIN code. You need to re-enter to confirm and the option Use PIN for purchases will then light up. Tap on the check box next to this mode and you are done. Nobody will then be able to spend money in apps or on the Google Play store without your consent. Return out of the settings menu and you are done.

To reverse it, head back to Settings in Google Play and tap on Unlock settings. You will need to re-enter your PIN code and then you can tap once more on Use PIN for purchases to unblock.

Have you ever had in-app purchase troubles? Let us know in the comments below...