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.

EU watchdogs announced under the bloc's Digital Markets Act that Apple must step into line with strict new laws on making operating systems fully functional with other technologies. The Brussels-based authority gave the company six months to comply, or face the threat of future penalties.

While the announcement is a step shy of being a formal investigation, the EU aims to compel Apple to re-engineer its services to allow rival companies to access the iPhone's and iPad operating systems.

"Today is the first time we use specification proceedings under the DMA to guide Apple towards effective compliance with its interoperability obligations," EU competition chief Margrethe Vestager said in a statement. "Effective interoperability, for example with smartphones and their operating systems, plays an important role in this."

Cupertino, California-based Apple said it's created ways for developers to request additional interoperability with iPhone and iPad operating systems, while protecting users' security. Undermining protections built into its systems over time would put European consumers at risk, the company added.

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