Google is now facing off against the European Commission in a five-day court hearing to appeal a record $5.1 billion fine and a 2018 ruling that casts doubt on the viability of the tech giant’s Android OS business model.

In 2018, the European Union’s executive branch penalized Google €4.34 billion ($5.1 billion) for allegedly breaking antitrust regulations with its Android mobile operating system.

Google’s appeal to the EU General Court in Luxembourg, filed three years ago, is an attempt to overturn the punishment and the more serious penalty that came with it, which could alter how the company monetizes its free Android mobile operating system.

The main issue at the center of the case is Google’s contracts that compel Android device phone makers to pre-install Google Search and Google Chrome in order to license the Google Play Store, which is mandatory.

Google’s opposition to modified versions of its Android mobile operating system is also being challenged in court. If device makers build or sell devices based on a modified version of Android, Google bans them from licensing its proprietary apps, such as Search and the Play Store.

Google said that its business strategy, which keeps Android free, has benefited European manufacturers and developers significantly. Any changes to it could affect its ability to keep the software free, the company said. Google also argues that Android gives European customers more options, not less, and that Google is facing stiff competition from Apple’s iOS operating system.

“Android has created more choice for everyone, not less, and supports thousands of successful businesses in Europe and around the world. This case isn’t supported by the facts or the law,” Google said.

Analysts said the outcome of Google’s appeal in the EU could become a major precedent for similar cases that may be filed against the company in other countries.