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 first focus of the DMA compliance initiative announced today is iOS. In particular, the EU will scrutinize a set of iOS features that consumers use to link their iPhones with connected devices. Officials will inform Apple how to implement those features in a DMA-compliant manner.

European regulators want Apple to open up device pairing, notifications and more to other companies' products.