LONDON The boss of Britain's antitrust regulator said blocking Microsoft's acquisition of 'Call of Duty' maker Activision Blizzard was the right decision for the UK after both companies said it sent the wrong message to the global tech industry.

The Competition and Markets Authority (CMA) blocked the deal on Wednesday, saying it could hit competition in the nascent cloud gaming market.

Microsoft's president Brad Smith said the decision "had shaken confidence in the UK tech industry" and was "probably the darkest day in our four decades in Britain".

"If the government of the United Kingdom wants to bring in investment, if it wants to create jobs (...) it needs to look hard at the role of the CMA, the regulatory structure in the United Kingdom, this transaction, and the message that the United Kingdom has just said to the world," he told BBC radio.

"There's a clear message here - the European Union is a more attractive place to start a business than the United Kingdom," he added.

But CMA Chief Executive Sarah Cardell said the regulator's role was to make sure Britain was a competitive environment for businesses to be able to grow and thrive.

"That's important for UK consumers and UK business and it's those UK consumers and UK businesses that the CMA is here to protect," she told BBC radio.

Asked if the British government could override the block if it thinks it is damaging to the country, she said the CMA acted independently and it was accountable for its decision.

"The decision that the CMA takes is an independent decision that we reached looking at an overall assessment of the impact of the deal on competition, and we think that is the right decision for the UK," she said.

She said Britain had not acted alone, noting that the U.S. Federal Trade Commission was also suing to block the deal.

(Reporting by Muvija M and Paul Sandle; editing by Sarah Young and Emelia Sithole-Matarise)