Google Inc is working on a new operating system for inexpensive computers in a daring attempt to wrest away Microsoft Corp’s long-running control over people’s computing experience.

The new operating system, announced late Tuesday night on Google’s Web site, will be based on the company’s nine-month-old Web browser, Chrome. Google intends to rely on help from the community of open-source programmers to develop the Chrome operating system, which is expected to begin running computers in the second half of 2010. The Mountain View, California-based company disclosed its plans for the operating system shortly after an online technology news service, Ars Technica, and The New York Times telegraphed the news on their Web sites.

Shares of Google jumped $3.21 to $399.84 in premarket trading Wednesday, while Microsoft fell 18 cents to $22.35. Google is designing the operating system primarily for “netbooks,” a lower-cost, less powerful breed of laptop computers that is becoming increasingly popular among budget-conscious consumers primarily interested in surfing the Web.