LONDON — Britain’s top competition regulator on Wednesday moved to block Microsoft’s acquisition of video game publisher Activision Blizzard.

The measure marks a major blow for the U.S. tech giant, as it seeks to convince authorities that the deal will benefit competition.

Microsoft said it plans to appeal the decision.

Shares of Activision Blizzard slumped 10% in U.S. pre-market.

The U.K. Competition and Markets Authority said that it opposed the deal as it raises competition concerns in the nascent cloud gaming market.

Microsoft could make Activision’s games exclusive to its cloud gaming platform, Xbox Game Pass, cuting off distribution to other key players in the space, the CMA said.

Cloud gaming is a technology that enables gamers to access games via companies’ remote servers — effectively streaming a game like you would a movie on Netflix. The technology is still in its infancy, but Microsoft is betting big on it becoming a mainstream way of playing games.

“Allowing Microsoft to take such a strong position in the cloud gaming market just as it begins to grow rapidly would risk undermining the innovation that is crucial to the development of these opportunities,” the CMA said in a press release Wednesday.

Microsoft offered the CMA remedies in an attempt to resolve its concerns — including “requirements governing what games must be offered by Microsoft to what platforms and on what conditions over a ten-year period” — however the regulator rejected the proposals.

“Given the remedy applies only to a defined set of Activision games, which can be streamed only in a defined set of cloud gaming services, provided they are purchased in a defined set of online stores, there are significant risks of disagreement and conflict between Microsoft and cloud gaming service providers, particularly over a ten-year period in a rapidly changing market,” the CMA said.

Microsoft Vice Chair and President Brad Smith said in a statement that it remained “fully committed to this acquisition and will appeal.”

“The CMA’s decision rejects a pragmatic path to address competition concerns and discourages technology innovation and investment in the United Kingdom,” Smith said Wednesday.

“We have already signed contracts to make Activision Blizzard’s popular games available on 150 million more devices, and we remain committed to reinforcing these agreements through regulatory remedies. We’re especially disappointed that after lengthy deliberations, this decision appears to reflect a flawed understanding of this market and the way the relevant cloud technology actually works.”

Bobby Kotick, CEO of Activision Blizzard, told employees in a letter Wednesday that two companies have “already begun the work to appeal to the UK Competition Appeals Tribunal.”

“We’re confident in our case because the facts are on our side: this deal is good for competition,” he said. 

“At a time when the fields of machine learning and artificial intelligence are thriving, we know the U.K. market would benefit from Microsoft’s bench strength in both domains, as well as our ability to put those technologies to use immediately,” Kotick added. “By contrast, if the CMA’s decision holds, it would stifle investment, competition, and job creation throughout the UK gaming industry.” 

Microsoft announced its intention to acquire Activision Blizzard in January 2022 for $69 billion, in one of the biggest deals the video game industry has seen to date.

Executives at the Redmond, Washington-based technology giant believe the acquisition will boost its efforts in gaming by adding lucrative franchises like Call of Duty and Candy Crush Saga to its content offerings.

However, some of Microsoft’s competitors contested the deal, concerned it may give Microsoft a tight grip on the $200 billion games market. Of particular concern was the prospect that Microsoft may shut off distribution access to Activision’s popular Call of Duty franchise for certain platforms.

Sony, in particular, has voiced concern with Microsoft’s Activision purchase. The Japanese gaming giant fears that Microsoft could make Call of Duty exclusive to its Xbox consoles in the long run.

Microsoft sought to allay those concerns by offering Sony, Nintendo, Nvidia and other firms 10-year agreements to continue bringing Call of Duty to their respective gaming platforms.

Microsoft argues it wouldn’t be financially beneficial to withhold Call of Duty from PlayStation, Nintendo and other rivals given the licensing income it generates from keeping the game available on their platforms.

Microsoft President Brad Smith told CNBC last month that the company is offering Sony the same agreement as it did Nintendo — to make Call of Duty available on PlayStation at the same time as on Xbox, with the same features. Sony still opposes the deal.

The CMA had raised concerns with the potential for Microsoft to hinder competition in the nascent cloud gaming market via its Xbox Game Pass subscription service, which offers cloud gaming among its perks. Microsoft has committed to bring new Call of Duty titles to Xbox Game Pass on day one of its release.

Cloud gaming, or the ability to access games via PC or mobile devices over the internet, is still in its infancy and requires a strong broadband connection to work well. Cloud gaming made up only a fraction of global internet traffic in 2022.