European Union (EU) regulators accused Apple on Friday of distorting competition in the music streaming market, siding with Spotify in a case that could lead to a hefty fine and changes in the iPhone maker’s lucrative business practices.

The preliminary findings are the first time Brussels has levelled anti-competitive charges against Apple, although the two sides have had bruising clashes in the past, most notably a multibillion-dollar tax dispute involving Ireland, Reuters reported Friday.

Apple, Spotify and other parties can now respond. If the case is pursued, the EU could demand concessions and potentially impose a fine of up to 10 percent of Apple’s global turnover — as much as $27 billion, although it rarely levies the maximum penalty.

Apple found itself in the European Commission’s crosshairs after Sweden-based Spotify complained two years ago that the U.S. tech giant unfairly restricted rivals to its own music streaming service Apple Music on iPhones.

The EU competition enforcer, in its so-called statement of objections setting out the charge, said the issue related to Apple’s restrictive rules for its App Store that force developers to use its own in-app payment system and prevent them from informing users of other purchasing options, the Reuters report added.

European Competition Commissioner Margrethe Vestager said there were clear signs Apple’s App Store rules were affecting music streaming rivals’ business development and affecting app developers more widely.

“They (app developers) depend on Apple App Store as a gatekeeper to access users of Apple’s iPhones and iPads. This significant market power cannot go unchecked as the conditions of access to the Apple App Store are key for the success of app developers,” she told a news conference.

Vestager said Apple should end restrictive practices and refrain from doing anything that would replicate them. She also said other authorities were looking into the issue.

“We have contact with other jurisdictions doing similar cases, that could be the Dutch, the Australians, the Americans,” she said, adding she was also interested in the app gaming market, although it was early days.

Apple rebuffed the EU charge. “Spotify has become the largest music subscription service in the world, and we’re proud of the role we played in that,” the US company said in a statement.

“They want all the benefits of the App Store but don’t think they should have to pay anything for that. The Commission’s argument on Spotify’s behalf is the opposite of fair competition,” the Apple statement added.

Spotify welcomed the EU move, describing it as “a critical step toward holding Apple accountable for its anticompetitive behavior, ensuring meaningful choice for all consumers and a level playing field for app developers”.

Reuters was first to report about the imminent EU antitrust charge in March.