A UK watchdog is threatening action against video game app-makers it finds in breach of consumer protection laws.
The Office of Fair Trading (OFT) is concerned about in-game charges, saying it has seen evidence of "potentially unfair and aggressive commercial practices" after studying 38 popular titles. It has not said which they are.
Children might be particularly susceptible to such tactics, it warns.
As a consequence it has proposed new guidelines for developers.
These would apply to both apps and internet browser-based video games available via Facebook and elsewhere.
- Providing up-front information about the costs associated with a game before consumers download it
- Ensuring that gamers are not misled to believe they must make a payment to proceed if that is not the case, for example, if they could wait for a period of time instead
- Preventing the use of language or anything else that might exploit a child's inexperience, for example, implying an in-game character would be disappointed if they did not spend money
- Making it clear how to contact the business if the gamer has a complaint
- Only taking a payment if the account holder provides "informed consent", in other words a charge cannot be made because a password had recently been entered for something else
The OFT said some of the worst examples it had seen involved games that led children through an adventure but then withheld a promised reward until they spent money, and instances where the title made the player feel bad by telling them a virtual animal was "ill" but could be made better if the gamer made a purchase.
"I don't think children are always aware that when they click 'yes' it's spending money," Cavendish Elithorn, executive director at the OFT, told the BBC.
"Although parents can change their device settings to deal with some of that, many parents might not know, or it's only when they get the bill that they realise the setting was wrong.
"So, part of what we're keen to do is support parents in having the right tools to be aware of what their children are doing online."
He added that the OFT has the ability to take legal action against firms in the UK, and was working with partners in Europe, North America and Australia to try and get the same rules applied elsewhere.
So-called freemium games - where the original download is given away free of charge, but the player is encouraged to buy add-on items or services - were pioneered in Asian markets as a way to combat piracy.
They have since spread to the west with EA's Fifa 14, Disney's Where's My Water, King's Candy Crush Saga and Sega's Sonic Dash among popular titles to adopt the model.
Video games trade body Tiga - which had advised the OFT on the issue - said it found the guidelines encouraging.
"Tiga understands both the legislative responsibilities and concerns of the OFT, and the daily realities of making games in the UK today and around the world," said the organisation's chief executive Richard Wilson.
"I'm pleased to say the OFT and UK games business is leading the way in addressing these issues and helping build a sustainable future for this high tech, highly skilled, global industry."
The Association for United Kingdom Interactive Entertainment (Ukie) said it was useful to have clarity about the OFT's interpretation of the law, but added a note of caution.
"It is vital that any final guidelines, whilst primarily considering the best interests of children, do not inadvertently isolate UK consumers from accessing the games that they want to play, stifle the creativity of games developers or prevent the growth of the UK games industry," said chief executive Jo Twist.
"Consumers are now often able to download and play the latest games for free.
"In-app purchasing is optional within many of these games and is a way for millions of players to access the extra content that they want."
The OFT has invited interested parties to comment on its principles by 21 November.
It then intends to publish a final version of the guidelines by February and begin enforcement action in April.