The European Commission has accused Google of "abusing its position" in the apps market by promoting its own Android service at the expense of others.
What is Android?
Android is Google's mobile software that powers smartphones and tablets, that launched in 2007. It's open, and as a result is free for any manufacturer to use. This has seen it become the most widely used operating system in the world - found on Samsung, LG, HTC, Sony, Huawei and other devices.
What has the EC taken issue with?
The Commission says it believes Google has breached EU antitrust rules by imposing restrictions on device manufacturers and mobile network operators who wish to use the Android operating system, obliging them to install Google Search and Google Chrome - the company's web browser service - if they want to pre-install the Google Play app store on their own devices. The app store is a key source of revenue and content for many, from which more than 50 billion apps have been downloaded.
The Commission claims that this is Google forcing users to build more Google apps into devices in order to get a better Android experience, something they say is not fair to competing apps and services.
The Commission also alleges that manufacturers were barred from selling devices carrying competing software that was built using Android's open-source base code - which is freely available - and that financial incentives were offered to those who exclusively pre-installed the Google Search app on their devices.
It adds that it believes Google's conduct had denied consumers access to "innovative smart mobile devices based on alternative, potentially superior, versions of the Android operating system".
What has been Google's response?
The company's senior vice president and general counsel of Google Kent Walker said: "Android has helped foster a remarkable - and, importantly, sustainable - ecosystem, based on open-source software and open innovation. We look forward to working with the European Commission to demonstrate that Android is good for competition and good for consumers."
In a further blog post on the subject, Google argued its agreements were entirely voluntary and that anyone can "use Android without Google".
"You can download the entire operating system for free, modify it how you want, and build a phone. And major companies like Amazon do just that."
The company added that pre-loading apps helps to make new phones more stable.
"Manufacturers who want to participate in the Android ecosystem commit to test and certify that their devices will support Android apps. Without this system, apps wouldn't work from one Android device to the next.
"Imagine how frustrating it would be if an app you downloaded on one Android phone didn't also work on your replacement Android phone from the same manufacturer."
What happens next?
The technology firm has 12 weeks to respond and a protracted legal case is likely to follow. If found guilty of claims, it could be fined or forced to change its practices.