PARIS, May 13 (Reuters) - The European Commission said

on Monday it had designated hotel reservation website

Booking.com as a "gatekeeper", which subjects it to strict EU

obligations, and has opened an investigation to further

determine the regulatory status of social media network X.

Online advertising services X Ads and TikTok Ads as such did

not qualify as gatekeepers under the EU Digital Markets Act

(DMA), 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 Booking.com fell under the DMA -

in line with the company's own view - it had opened a market

investigation to further assess the rebuttal lodged by X,

opposing the gatekeeper 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 is one of the most stringent

regulations targeting tech giants' market dominance, imposing

tougher obligations to moderate content, allow fair competition

and make it easier for consumers to switch between services.

It designates companies with more than 45 million monthly

active users and 75 billion euros ($81 billion) in market

capitalisation as gatekeepers, providing a core platform service

for business users.

Following its designation as gatekeeper, Booking.com now

has six months to submit a detailed compliance report, the

Commission said. However, some DMA rules are applicable with

immediate effect, notably the obligation to inform the

Commission of any intended concentration in the digital sector.

If a gatekeeper does not comply with the DMA, the

Commission can impose fines of up to 10% of the company's total

worldwide turnover, which can go up to 20% in case of repeated

infringements. It can also impose limits on takeover activity.

Alphabet, Amazon ( AMZN ), Apple ( AAPL ),

Meta and Microsoft ( MSFT ) were designated gatekeepers

last year.

From March 7, they must make their messaging apps

interoperable with rivals and let users decide which apps to

pre-install on their devices. They are also not allowed to

favour their own services over rivals' or prevent users from

removing pre-installed software or apps.