The European Union says it will outline steps to compel Apple to open up its iPhone and iPad operating systems to competing technologies under the bloc’s tough new digital rulebook, which threatens hefty fines for breaches.

Apple Inc. has been warned by the European Union to open up its highly guarded iPhone and iPad operating systems to rival technologies, or eventually risk significant fines under its flagship digital antitrust rules.

The European Union's enforcement arm said Thursday it's initiating a pair of "specification proceedings" into whether Apple is complying with its "interoperability obligations" under the Digital Markets Act.