Spotify has accused Apple of “causing grave harm to Spotify and its customers” by rejecting an update to Spotify’s iOS app, Spotify’s legal chief wrote in a letter to his counterpart at Apple, Recode reports. In a June 26 letter to Apple general counsel Bruce Sewell, Spotify general counsel Horacio Gutierrez complained of behavior he described as harmful to competition, namely Apple’s insistence that Spotify use Apple’s billing system, from which Apple receives a cut of monthly subscriptions.

“This latest episode raises serious concerns under both U.S. and EU competition law,” Gutierrez reportedly wrote. “It continues a troubling pattern of behavior by Apple to exclude and diminish the competitiveness of Spotify on iOS and as a rival to Apple Music, particularly when seen against the backdrop of Apple’s previous anticompetitive conduct aimed at Spotify … we cannot stand by as Apple uses the App Store approval process as a weapon to harm competitors.”

Spotify shared copies of the letter with some legislative staffers in Washington, D.C., according to Recode.

Spotify declined to comment to Pitchfork. Pitchfork has also reached out to Apple for comment.

The report of the letter came a day after Senator Elizabeth Warren criticized Apple Music as an example of concentrated corporate power that can reduce competition. She said the Federal Trade Commission was investigating Apple’s treatment of competitors’ music streaming services for potential violations of antitrust law.

Spotify’s global head of communications and public policy, Jonathan Prince, said at the time, “You know there’s something wrong when Apple makes more off a Spotify subscription than it does off an Apple Music subscription and doesn’t share any of that with the music industry.”