Epic Games, the studio behind the hit game Fortnite, has started legal action against Apple and Google in the UK.
Both Apple and Google removed Fortnite from their smartphone app stores in a row over the cut of sales they take from developers.
Epic Games claims the two tech giants' rules are in violation of UK competition laws.
But both companies say Epic Games broke their terms and conditions by trying to bypass their payment systems.
Apple said Epic Games' action had been "reckless".
What is the dispute about?
When people buy an app, subscription or in-app item on Apple's App Store or the Google Play store, the tech giants take a cut of the fee.
Typically they take a 30% cut, although in some cases the rate is lower.
For this reason, some companies charge more for subscriptions when people buy directly in the app instead of on their website, while others do not sell subscriptions in-app at all. Companies are also banned from telling app users a cheaper deal is available.
Epic Games has previously said it thinks a 30% cut is too much, given the amount of money and effort it invests in developing its games.
In August 2020, the company bypassed the Apple and Google payment systems to let Fortnite players buy virtual currency at a lower price, direct from Epic.
Both app stores then removed Fortnite from their services, and Apple terminated Epic Games' developer account.
What is the UK case about?
While Apple produces iOS exclusively for its iPhones, Google's Android software is open-source so in theory any smartphone-maker can use it.
However, Google also produces a suite of proprietary tools including Maps, Search and the Play Store, which power some of the most popular services on smartphones. Phone-makers must enter an agreement with Google to use them on their devices.
Epic Games alleges that both companies abuse their power by:
unfairly restricting competition from alternative app stores
stopping developers using their own payment-processing services for purchases
charging "unfair prices for the distribution of apps"
Epic Games is not seeking a financial payout but instead wants the court to order Apple and Google to:
reinstate Epic Games' software and accounts
let developers use their own payment-processing systems in their apps
let consumers download Epic Games' software outside of the App Store or Google Play
In a statement, Epic Games said: "We believe that this is an important argument to make on behalf of consumers and developers in the UK and around the world who are impacted by Apple and Google's misuse of market power."
It has taken similar legal action in the US and Australia.
On Thursday, 21 January, the CTA will decide whether Epic Games can include US-based divisions of Apple and Google in its legal action.
How have Apple and Google responded?
Apple said its priority was to create a "safe and trusted place to download software" and that it applied its rules equally to all developers.
"Epic enabled a feature in its app which was not reviewed or approved by Apple, and they did so with the express intent of violating the App Store guidelines," it said in a statement.
"Their reckless behaviour made pawns of customers, and we look forward to making this clear to courts in the UK."
Google said: "The open Android ecosystem lets developers distribute apps through multiple app stores. For game developers who choose to use the Play Store, we have consistent policies that are fair to developers and keep the store safe for users."
It said it welcomed the opportunity to "continue discussions" with Epic Games about bringing Fortnite back to the Google Play store.