The Office of Fair Trading has announced that developers have until April 1 to ensure their apps adhere to its in-app purchase principles.
A list of proposed principles was introduced last September and the OFT is expected to publish the finalised list later today.
Developers who do not follow the rules will be in breach consumer protection law and could risk enforcement action.
Among the rules will be a requirement for developers to tell consumers upfront about costs associated with a game or in-game advertising, as well as any important information such as whether their personal data is to be shared with other parties for marketing purposes.
There is also a rule stating that in-app purchases must not be taken unless the payment account holder, such as a parent, has given their express, informed consent.
The OFT is also due to publish a list of guidelines for parents that will help them ensure their child isn't pressured into making in-app purchases.
The guidelines include changing the payment options setting on their device so that a password is required for every purchase, checking in advance to see whether a game has any in-app purchases, seeing the game contains social elements and playing the game themselves to understand what their children will see.
They are also advised to regularly check that their child is happy playing a game, as content can change via automatic updates.
The OFT first warned developers in September that they will face legal ramifications over the implementation of aggressive in-app purchases unless they adapt their existing business practices.
Earlier this month the FTC in America ordered Apple to refund $32.5 million to parents whose children made in-app purchases without their consent.