The latest tech deal, Microsoft's second biggest after its $26.2 billion LinkedIn purchase in 2016, follows heightened regulatory scrutiny of "killer acquisitions" whereby tech giants buy and shut down nascent start-ups and potential rivals.

Microsoft announced the Nuance deal, which would boost its presence in cloud services for healthcare, in April and it has already received regulatory approval in the United States and Australia, without remedies given

The European Commission, which is scheduled to decide on Microsoft's proposed Nuance purchase by December 21, but word on the street it is a done deal.

Microsoft has been in preliminary discussions with Britain's CMA antitrust agency ahead of a formal request for approval of the Nuance deal, the sources said. The company's request is expected to be filed in January.

The European Commission last month asked customers and competitors to list concerns about the deal, such as whether Microsoft could favour Nuance over its competitors or unfairly bundle its offerings.

Tech companies have ramped up acquisitions of AI-focused firms to try to stay ahead of competitors as more integrate this technology into their products and services.