The European Commission said it had designated Booking as a so-called "gatekeeper" under the Digital Markets Act (DMA), while also opening a market investigation into the social media network X to further determine the platform's regulatory status.
Online advertising services X Ads and TikTok Ads as such did not qualify as gatekeepers, the EU added.
The Commission said in March that Elon Musk's X, TikTok owner ByteDance, and Booking.com may meet EU criteria subjecting them to tough tech rules as so-called gatekeepers.
While the EU on Monday said that Booking qualified as a gatekeeper, it had opened a market investigation to further assess the rebuttal lodged by X, which opposes such a status.
ByteDance was labelled a gatekeeper in July last year but TikTok has challenged that at Europe's second-highest court.
The Digital Markets Act (DMA) is one of the most stringent regulations targeting large tech firms' market dominance. It puts companies under tougher obligations to moderate content, allow fair competition and make it easier for consumers to switch between services.
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