Microsoft’s president Brad Smith has criticized the UK Competition and Markets Authority’s (CMA) decision to block the tech giant’s acquisition of Activision Blizzard, stating that it had “shaken confidence” in the UK as a destination for tech businesses. The CMA blocked the acquisition on Wednesday, citing concerns about its impact on competition in the nascent cloud gaming market.

In response, Smith said that the decision was “probably the darkest day in our four decades in Britain,” and that it sent the wrong message to the global tech industry about the UK. He argued that if the UK government wants to attract investment and create jobs, it needs to look hard at the role of the CMA and the regulatory structure in the UK. Smith also criticized the CMA’s handling of the matter, claiming that Microsoft had answered the regulator’s questions and had told them to come back with any more concerns, but had received no response.

A spokesman for British Prime Minister Rishi Sunak pushed back against Smith’s comments, stating that the UK has an extremely attractive tech sector and a growing games market, and that the government will continue to engage proactively with Microsoft and other companies.

CMA Chief Executive Sarah Cardell defended the regulator’s decision, stating that its role was to ensure that the UK remained a competitive environment for businesses to grow and thrive. She also noted that the US Federal Trade Commission was also pushing for the deal to be blocked on competition grounds.

Microsoft has said that it will appeal the decision, with “aggressive” support from Activision. Appeals against CMA rulings are heard by the Competition Appeals Tribunal, which makes a judgment on the merits of the decision, but does not provide an opportunity for Microsoft to submit new remedies.

The decision to block the acquisition highlights the ongoing tensions between tech giants and regulators around the world, as governments seek to ensure that the tech sector operates in a fair and competitive environment.