In a written message to employees, Panetta said that he notified members of Congress Wednesday that if the White House and Congress cannot strike a deficit reduction deal before March 1 to avoid the furloughs, all affected workers will get at least 30 days' advance notice.
The furloughs would be part of broad spending cuts the Pentagon would implement in order to achieve $46 billion in reductions through the end of this budget year, which ends Sept. 30. More cuts would come in future years as long as the automatic government spending cuts, known as sequestration, remained in effect.
"In the event of sequestration we will do everything we can to be able to continue to perform our core mission of providing for the security of the United States, but there is no mistaking that the rigid nature of the cuts forced upon this department, and their scale, will result in a serious erosion of readiness across the force," Panetta wrote.
Pentagon officials have said the furloughs would be structured so that nearly all 800,000 workers lose one day of work per week for 22 weeks, probably starting in late April. That means they would lose 20 percent of their pay over that period.
The Pentagon has begun discussing details of the furloughs with defense worker union officials.
President Barack Obama has exempted military personnel from furloughs.
The only civilian Pentagon workers who would be exempt from furloughs would be Senate-confirmed political appointees such as the defense secretary and deputy defense secretary, as well as a relatively small number of workers deemed essential to protect the safety of defense property and personnel.
Panetta said the administration is still working with Congress to avoid automatic budget cuts by reaching agreement on a deficit reduction plan.