Microsoft has filed lawsuits against Google and the employee, Dr Kai-Fu Lee, in respect of confidentiality and non-competition agreements.
The software giant claims to have contacted Google on the matter but received no reply.
Google has already named Dr Lee as its intended head of a research and development unit in China, which is due to launch later this year.
Dr Lee was on the team that developed Microsoft's search engine released in January in direct competition to Google.
Microsoft lawyer Tom Burt said in a briefing to journalists yesterday that the company was aware of Dr Lee's move, but did not know the exact nature of his new role until it saw Google's press release.
He added that, although Dr Lee resigned on Monday, he did not negotiate the necessary release from contract that normally occurs.
Burt went on to describe Dr Lee as being among a few executives who had been privy to highly confidential details about Microsoft's business.
A Microsoft spokesman said: "Creating intellectual property is the essence of what we do at Microsoft, and we have a responsibility to our employees and shareholders to protect our intellectual property.
"As a senior executive, Dr Lee has direct knowledge of Microsoft's trade secrets concerning search technologies and China business strategies.
"He has accepted a position focused on the same set of technologies and strategies for a direct competitor in egregious violation of his explicit contractual obligations."
Dr Lee recently moved to the Redmond campus as a corporate vice president. He is renowned for his work in speech recognition technology and was most recently head of Microsoft's natural interactive services division which focuses on ease of use of computers.
Confidentiality and non-competition agreements typically bar 'sensitive' employees from joining competitors for a set period of time, or from hiring other employees, or from disclosing the company's trade secrets.
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