British antitrust regulators on Wednesday blocked Microsoft's $69bn purchase of video game maker Activision Blizzard, thwarting the biggest tech deal in history over worries that it would stifle competition for popular titles like Call of Duty in the fast-growing cloud gaming market. The Competition and Markets Authority said in its final report that "the only effective remedy" to the substantial loss of competition "is to prohibit the Merger." The companies have pledged to appeal. The all-cash deal announced 15 months ago faced stiff opposition from rival Sony, which makes the PlayStation gaming system, and also was being scrutinised by regulators in the United States and Europe over fears that it would give Microsoft and its Xbox console control of hit franchises like Call of Duty and World of Warcraft. The UK watchdog's decision "came as a surprise to most people" and heightens global uncertainty over the deal, said Liam Deane, a game industry analyst for research firm Omdia.