Apple and Spotify have engaged in a verbal feud over comments made by the co-founder of Spotify, Daniel Ek.

Ek criticized Apple’s App Store for stifling innovation, development, and revenue for third party app creators. Ek, who is also the CEO of Spotify, made a formal complaint with the E.U. anti-trust watchdog, the European Commission. This will most likely lead to an investigation which will look into Apple’s developer requirements and determine whether or not they are artificially incentivizing users to go with Apple Music over other music streaming apps by making the competition more expensive.

According to a blog post by Ek, the “Apple tax” gives Apple 30% of the in-app revenue from third party developers, which makes the cost of their premium monthly memberships go up. This, he argues, would encourage users to instead use Apple Music, and create an environment that suppresses non-Apple developers and discourages competitors.

On the other side, Apple spokespeople came out against those accusations, stating that their cut of the profits goes towards maintaining a functioning platform on which Spotify and others can do their business. They also called out the company for making the accusations when they consider the App Store to be one of the major reasons that Spotify is a multi-billion dollar corporation, mentioning that they most likely would not have gotten to that point without the App Store and Apple products as a platform.