The UK competition regulator has blocked Microsoft’s $75bn acquisition of the Call of Duty maker Activision Blizzard, in a possibly fatal blow to the software giant’s biggest-ever deal.

The Competition & Markets Authority said on Wednesday it believed the company would be commercially motivated to make Activision’s games exclusive to its own cloud gaming service.

Activision said the ruling “contradicts the ambitions of the UK to become an attractive country to build technology businesses” and was a “disservice to UK citizens, who face increasingly dire economic prospects”.

In an email to employees on Wednesday chief executive Bobby Kotick said the decision was “far from the final word on this deal.” He said: “If the CMA’s decision holds, it would stifle investment, competition, and job creation throughout the UK gaming industry.

Activision’s shares fell 11 per cent in pre-market trading.

The CMA argued that Microsoft had failed to address its concerns about protecting innovation in the nascent cloud gaming market.

But Brad Smith, Microsoft vice-chair and president, said his group “remains fully committed to this acquisition and will appeal”, warning that the decision “discourages technology innovation and investment in the United Kingdom”.

He added that the CMA’s decision reflected “a flawed understanding of this market and the way the relevant cloud technology actually works”.

The ruling comes after the regulator retreated from a key concern in March, in a step that had appeared to pave the way for the two sides to inch the deal through. The companies had hoped to reassure the CMA that licensing deals signed with cloud gaming platforms would be sufficient to appease the watchdog.