The EU on Monday said Apple's operating system for iPads must comply with tougher new rules that Brussels is imposing to rein in the world's biggest digital companies.

The European Commission designated Apple's iPadOS system as a "core" service under the landmark Digital Markets Act (DMA), which forces companies to modify their business ways to encourage competition between online platforms.

It joins other Apple products that were already in the DMA net since September: iOS for iPhones, the App Store, and the Safari browser.

Under the DMA, digital firms and some products called "gatekeepers" have to abide by a list of rules including allowing interoperability with rivals' communication services and limiting how data is shared between products put out by the same parent company.

Other designated gatekeepers are Google parent Alphabet, Amazon, TikTok owner ByteDance, Meta and Microsoft.

The inclusion of iPadOS adds to a long tussle between the European Union and Apple over the bloc's new digital laws.

Apple has been one of the DMA's most vocal public critics, claiming the law ushers in privacy and security threats for users.

The commission, the EU's powerful competition regulator, said it named the iPadOS system because it locked users into the iPad operating system.

"Apple leverages its large ecosystem to disincentivise end users from switching to other operating systems for tablets," it said.

The operating system also "locked-in" Apple's business users, it said, "because of its large and commercially attractive user base, and its importance for certain use cases, such as gaming apps".

Apple has six months to comply with the DMA gatekeeper rules, the commission said in a statement.

"Today's decision will ensure that fairness and contestability are preserved also on this platform, in addition to the 22 other services we designated last September," the EU's competition commissioner, Margrethe Vestager, said.

Apple already faces a commission investigation under the DMA.

In March, Brussels said it would probe whether Apple's App Store allows developers to present users with offers outside of its app marketplace, free of charge.