Companies offering free web or app-based games will have to provide information on in-game marketing to children including upgraded membership or virtual currency such as coins, gems or fruit. Typically, players can access only portions of these games for free, with new levels or features, such as faster game play, costing money.

The OFT is also asking for parents and consumer groups to contact it with any information about potentially misleading or commercially aggressive practices.

One parent told MoneySavingExpert.com his seven-year-old daughter had managed to rack up £69.99 on the College Girl app, which he only noticed when he received an iTunes receipt.

In another example, a five year old spent £65 on in-app purchases on an iPad.

According to Ofcom, in 2012, 28% of children aged five to 15 owned smartphones, an increase from 20% in the previous year.

The OFT investigation will explore whether these games are misleading, commercially aggressive or otherwise unfair.

As part of the investigation, the OFT will also consider whether the full cost of some of these games is made clear when they are downloaded or accessed, potentially leading to children and parents to make decisions they may not have made if prices were more transparently advertised at the start of the purchasing process.

Cavendish Elithorn, OFT senior director for goods and consumer, said:

“We are concerned that 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. We are speaking to the industry and will take enforcement action if necessary.”