The Financial Times first reported the story on Friday, and Business Insider publishing a full copy of the letter on Monday.
The European tech firms did not specifically name Google or Apple in the letter but referenced mobile operating systems, app stores, search engines, marketplaces and social media services as examples of the kind of platforms they take issue with.
In one part of the letter, the CEOs said examples of platforms turning into gatekeepers include them "restricting access to data or interaction with consumers, biased ranking and search results to lack of clarity, imbalanced terms and conditions and preference of their own vertically integrated services."
With Google dominating the search market in Europe and its Android mobile operating system along with Apple's iOS making up over 90 percent of the mobile market in the region, it's clear who the chief executives are singling out.
Apple and Google were unavailable for comment when contacted by CNBC. The European Commission was also unavailable.
The letter was sent to the commission ahead of a midterm review of the Digital Single Market, a policy aimed at harmonized rules across the bloc when it comes to policy around the digital economy. The review is scheduled for Wednesday.
Apple and Spotify have clashed in the past. Last year, Spotify sent a letter to Apple accusing it of "causing grave harm" to the company and its customers by rejecting an update to the streaming service's iOS app. Spotify has also hit out against the 30 percent charge Apple has on payments made via iTunes. In July, the Swedish firm sent an email to remind customers it was cheaper to sign up to Spotify directly through the company's website than via the App Store.