Angry parents in America have received refunds from Apple after their children ran up credit card bills playing a free iPhone download.

The Smurfs’ Village game became the highest grossing application in the iTunes store, despite being free to download, after users used real money to buy in-game items like Smurfberries that speed up gameplay

A wheelbarrow of Smurfberries costs $59.99 (£38) and can be bought by inputting the user’s iTunes password.

However, if the password has been inputted within the past 15 minutes, further purchases do not require it – a loophole that means children are able to wrack up enormous bills in a matter of minutes.

And according to users the “15-minute password-free period”, which is set by Apple, does not always work, with some parents saying hours and even days can pass without purchases needing a password.

Apple introduced “in-app purchases” last year, letting developers use the iTunes billing system to sell items and add-ons in their games and applications.

This year, developers have started to use the system in earnest as the main revenue stream for many games. Of the 10 highest-grossing apps in the App Store, six are games that are free to download but allow in-app purchases. TeamLava LLC, the publisher of Bakery Story and Farm Story, says the games follow all of Apple’s rules and restrictions. In either game, it’s possible to buy $99.99 (£63) worth of “gems” in one go.

Apple spokeswoman Trudy Muller says the password system is adequate and points out that parents can restrict in-app purchases. Some parents have received refunds from Apple after complaining.