The European Union may soon escalate its probe into how Apple Inc. allegedly squeezes rival music-streaming service Spotify Technology SA, according to three people with knowledge of the case.

The European Commission is working on a so-called statement of objections laying out antitrust issues into Apple’s treatment of Spotify on its App Store, the people said, on condition of anonymity. While regulators have been gathering information for the document in recent weeks, it isn’t likely to be sent imminently, two people said. One person said it could come as soon as March.

The probe is the most advanced among EU investigations that are looking into Apple’s payment system and e-books service and adds to a growing backlash against the levies Apple and Alphabet Inc.’s Google charge outside developers for using their app stores. The U.K. opened its own investigation into Apple’s in-app purchases earlier Thursday and the U.S. is also scrutinizing the issue.

Spotify complained in 2019 that Apple unfairly squeezes its music streaming service with ever-changing rules and a large sales commission on the App Store. It said it was forced to “artificially” increase monthly subscriptions for its premium service to cover the extra costs.

Regulators around the world are examining how companies such as Apple play a “gatekeeper” role by creating and policing rules how others that dependent on them, such as app developers, can use their online platforms. The EU drafted rules in December that would make it easier for enforcers to crack down on potential abuses.

Apple declined to comment. Spotify didn’t immediately respond to a request for comment. The European Commission also declined to comment.

Reuters reported earlier Thursday on potential antitrust objections in the investigation.