Britain’s top competition regulator moved to block Microsoft’s $69 billion acquisition of video game publisher Activision Blizzard.

The measure marks a significant blow for the US tech company, as it seeks to convince authorities that it will benefit competition. Microsoft said it will appeal the decision.

Shares of Activision Blizzard declined more than 8% in early US trading.

The UK Competition and Markets Authority (CMA) said it opposed the deal since it raises competition issues in the infant cloud gaming market.

The CMA said, “Microsoft could make Activision Blizzard’s games exclusive to its cloud gaming platform, Xbox Game Pass, cutting off distribution to other key industry players.”

The CMA added, “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.”