British antitrust regulators on Wednesday blocked Microsoft’s plans to acquire the video game giant Activision Blizzard for $69 billion, a significant hurdle for what would be the largest consumer tech acquisition since AOL bought Time Warner two decades ago.

The Competition and Markets Authority in Britain said in a statement that Microsoft’s proposal “failed to effectively address the concerns in the cloud gaming sector.”

The decision bolsters an effort by the Federal Trade Commission to block the acquisition and is a red flag for big technology companies trying to make large deals despite increasing government scrutiny over whether they abuse their power to hurt rivals and consumers.

“Microsoft already enjoys a powerful position and head start over other competitors in cloud gaming and this deal would strengthen that advantage giving it the ability to undermine new and innovative competitors,” Martin Coleman, the chair of a panel that conducted an investigation for the C.M.A., said in a statement.

A spokesman for the F.T.C. did not immediately respond to a request for comment.

Microsoft said it would appeal the ruling.

“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,” Brad Smith, Microsoft president, said in a statement.