A US federal judge ordered the Microsoft Corporation to split into two companies today, prescribing the biggest corporate breakup since AT&T while harshly rebuking the global software giant for stifling computer-age competition.

Potentially monumental, the ruling by US District Judge Thomas Penfield Jackson was not the last word in a case that could define the limits of companies operating in a high-tech economy. Microsoft has promised to appeal in the case, which had been pressed by the Justice Department and 19 states. The case could go to the US Court of Appeals or directly to the Supreme Court.

Jackson, who concluded two months ago that Microsoft had violated antitrust law, ordered the company to be split into these parts:

 One that would oversee the Windows operating system.

 A second that would handle all other Microsoft software, such as its "Word" program.

The Justice Department and 17 of the 19 states recommended that the company be broken into two pieces - one to own and market Windows, source of the company's monopoly position, and another to control other software and the company's Internet business.