Microsoft Corp's new phone software will be built on the same core as its new upcoming PC and tablet operating system, bringing the software company one step closer to unifying its Windows franchise across a full range of screens that are revolutionizing computing.

The world's largest software company, which is running to keep up with Apple Inc's iPhone and Google Inc's Android devices, said the common core means customers will have a greater choice of phones and applications, and be able to switch between multiple devices more easily.

The move follows the launch of the Surface tablet on Monday, Microsoft's effort to join the fast-growing mobile computing market and to tackle Apple's iPad head on.

At an event in San Francisco on Wednesday, Microsoft officially announced its new phone software, called Windows Phone 8, and said phones running the software would hit the market this autumn.