SO-CALLED "free" web and app-based children's games are under investigation after concerns users can run up substantial costs.

The Office of Fair Trading (OFT) said it was investigating whether children were being unfairly pressured or encouraged to pay for content in free games, such as upgraded membership or virtual currency in forms including coins, gems or fruit. Typically, players can access only certain areas of these games for free and must pay for higher levels or features.

The OFT has written to firms who offer such games asking them for data on how they market to children and is asking parents and consumer groups for information about potentially misleading or commercially aggressive practices.

The OFT's investigation will look into whether these games include "direct exhortations" to children, or strong encouragement to make a purchase, to do something that will require making a purchase, or to persuade an adult to make a purchase for them. It will also consider whether the full cost is made clear when they are downloaded or accessed.

The OFT senior director for goods and consumer, Cavendish Elithorn, said: "We are concerned children and their parents could be subject to unfair pressure to purchase when they are playing games they thought were free, but which can actually run up substantial costs.

"The OFT is not seeking to ban in-game purchases, but the games industry must ensure it is complying with the relevant regulations so that children are protected."