Photo courtesy of SpotifyTough week for Apple Music! Earlier this week, Senator Elizabeth Warren dropped a hot 16 against the streaming giant when she called into question the business practices of Google, Amazon, and Apple, specifically singling out Apple Music for their overall propensity to "snuff out competition."A Spotify spokesman was quick to back up Warren's tough talk, and now Spotify's doubling down with incendiary claims that Apple is purposefully rejecting an updated version of Spotify's app from the App Store. The company sent a letter to Apple's top lawyer earlier this week, claiming that Apple is "causing grave harm to Spotify and its customers" by doing so."This latest episode raises serious concerns under both US and EU competition law," Spotify general counsel Horacio Gutierrez wrote in the letter, which Recode reports has also been circulated to some Congressional staff. "It continues a troubling pattern of behavior by Apple to exclude and diminish the competitveness 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."Recode also notes that Spotify is way, way ahead of Apple Music when it comes to subscribers, with a healthy 30 million count doubling Apple's 15 million. (Granted, 15 million is nothing to sneeze at either—especially when you consider that Tidal, which launched before Apple Music, has only garnered 3 million subscribers as of March of this year.) Still, if Apple is purposefully keeping Spotify from introducing an updated version of their app, that's pretty shady—and it adds some fuel to the fire that the FTC's current investigation of Apple Music is generating, too.Follow Larry Fitzmaurice on Twitter.