SAN FRANCISCO  In a direct challenge to Microsoft, Google announced late Tuesday that it is developing an operating system for PCs that is tied to its Chrome Web browser.

The software, called the Google Chrome Operating System, is initially intended for use in the tiny, low-cost portable computers known as netbooks, which have been selling quickly even as demand for other PCs has plummeted. Google said it believed the software would also be able to power full-size PCs.

The move is likely to sharpen the already intense competition between Google and Microsoft, whose Windows operating system controls the basic functions of the vast majority of personal computers.

“Speed, simplicity and security are the key aspects of Google Chrome OS,” said Sundar Pichai, vice president of product management, and Linus Upson, engineering director, in a post on a company blog. “We’re designing the OS to be fast and lightweight, to start up and get you onto the Web in a few seconds.”