Detlef Eckert is taking a three-year sabbatical from the European Commission to work in the Trustworthy Computing division of Microsoft Europe, said a Commission spokesman.
The Commission insisted that Eckert would not be working at Microsoft in any area relating to the ongoing investigation of the software giant's business practices.
This was repeated by a Microsoft. "He will not have anything to do with the European antitrust case," a company representative insisted.
Eckert has been the head of a unit within the Information Society directorate of the Commission that deals with analysis, policy planning and the Commission's e-Europe initiative. Most recently he has been involved in policy making for the rollout of broadband Internet access across Europe.
The Commission is currently investigating allegations that Microsoft may have violated antitrust rules by using illegal practices to extend its dominant position in the market for personal computer operating systems into the market for low-end server operating systems.
Microsoft is also accused of bundling some so-called middleware functions, such as its Media Player software, into Windows 2000.
The Commission has said it expects to issue a preliminary ruling by the end of this year, with a final verdict due early next year.