While the letter named no names, it is being widely speculated that these companies had Apple and Google in mind.

The issue, which has been percolating for some time, is that Google (GOOGL) and Apple (AAPL) offer the operating systems that run the vast majority of mobile devices and also field their own online music streaming services, which compete with Spotify and others.

Business Insider subsequently published the letter signed by Spotify co-founders Daniel Ek and Martin Lortenzon, among others, and sent to Jean-Claude Juncker, president of the European Commission in Brussels.

The bottom line, according to these firms: If the those two software giants favor their own streaming services over those of rivals, it hurts competition and consumer choice. An excerpt from the letter reads: “A key factor to consider is the role of platforms as gateways to consumers and as competitors to services in downstream markets and their ability to turn into gatekeepers.”

Other signatories included Hans-Holger Albrecht CEO of Deezer, a music streaming service out of Germany; Oliver Samwer, CEO and founder of e-commerce company Rocket Internet; and Ralph Dommermuth, CEO and founder of Internet service provider United Internet.

Related: Google Foe Takes Android Complaint to EU Regulators

In an earlier skirmish, Spotify claimed last summer that Apple blocked approval of its latest iPhone and iPad app solely to harm Spotify, a claim Apple denied. European regulators have long had issues with dominant U.S. tech companies including Google, Apple, Amazon (AMZN), Microsoft (MSFT), and others.

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These European companies are asking regulators to enact additional rules to ensure that Apple and Google provide a level playing field for other services.

Fortune contacted Spotify, Apple, and Google for comment and will update this story as needed.