Apple has bought a huge billboard at the world's biggest gadget show to troll the rest of the tech industry.

Towering over Las Vegas, where the CES 2019 technology show is being held, the ad points to Apple's privacy policies in an apparent dig at competitors such as Google.

"What happens on your iPhone stays on your iPhone," the message reads.

At the bottom of the board is a link to Apple's privacy website, which lays out the ways that "Apple products are designed to protect your privacy".

"At Apple, we believe privacy is a fundamental human right," it reads. "Every Apple product is designed from the ground up to protect that information. And to empower you to choose what you share and with whom."

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The ad appears to have been uniquely made to be shown off in Las Vegas, but mirrors some other marketing around sustainability that Apple has rolled out across the world. The privacy ad might make its way to other billboards in the future.

The CES conference has already seen a whole host of internet of things and other devices unveiled that could have privacy problems. Cyber security experts have repeatedly warned about the dangers of the kind of cheap and unsafe gadgets that are often shown off at the tech show – but hundreds of such new devices will nonetheless be shown off through the week.

Apple doesn't traditionally take part in the CES gadget show, though all of its competitors do and the iPhone casts a long shadow over the event.