Almost all of the world's phones run on Android or Apple's iOS
Some of Europe's leading tech firms - including Spotify - have warned the EU that they believe that US tech firms are manipulating markets.
While they are not named, Apple and Google are believed to be the companies in question.
The letter says that the developers of operating systems, app stores, and search engines who developed "gateways" for companies to grow are now acting as "gatekeepers" and giving their own products preferable treatment.
The signatures alleged that big US firms "can and do abuse their privileged position." More than 90% of the world's phones now run on either Apple's iPhone operating system or Google's Android.
These companies are calling for, "specific rules guiding the interactions between platforms and their business users," in the EU.
Spotify has previously accused Apple of stalling its app updates for iPhone users while it was promoting its rival service, Apple Music.
This call comes as the EU continues to draft its policies for a future 'digital' version of the Single Market.
In 2016, Google was accused of manipulating search results to promote its own products by the European Commission. That charge was rejected by the US firm.