The groundbreaking move recommended by the Joint Chiefs of Staff overturns a 1994 rule prohibiting women from being assigned to smaller ground combat units.
Officials briefed the Associated Press on condition of anonymity so they could speak ahead of the official announcement.
There long has been opposition to putting women in combat, based on questions of whether they had the necessary strength and stamina for certain jobs, or whether their presence might hurt unit cohesion.
But as news of Panetta's expected order got out, members of Congress, including the Senate Armed Services Committee chairman, Sen. Carl Levin (D., Mich.), announced their support.
"It reflects the reality of 21st-century military operations," Levin said.
Panetta's move comes in his final weeks as Pentagon chief and just days after President Obama's inaugural speech in which he spoke passionately about equal rights for all. The new order expands the department's action of nearly a year ago to open about 14,500 combat positions to women, nearly all of them in the Army. Panetta's decision could open more than 230,000 jobs, many in Army and Marine infantry units, to women.
Besides questions of strength and performance, there also have been suggestions that the American public would not tolerate large numbers of women being killed in war.
Under the 1994 Pentagon policy, women were prohibited from being assigned to ground combat units below the brigade level. A brigade is roughly 3,500 troops split into several battalions of about 800 soldiers each. Historically, brigades were based farther from the front lines and often included top command and support staff.
The necessities of combat in Iraq and Afghanistan, however, propelled women into jobs as medics, military police, and intelligence officers that were sometimes attached - but not formally assigned - to battalions. So while a woman couldn't be assigned as an infantryman in a battalion going out on patrol, she could fly the helicopter supporting the unit, or move in to provide medical aid.
Still, as recent surveys and experiences have shown, it will not be an easy transition. When the Marine Corps sought women to go through its tough infantry course last year, two volunteered, and both failed to complete the course. And there may not be a wide clamoring from women for the more intense, dangerous, and difficult jobs - including some infantry and commando positions.
In the Navy, however, women have begun moving into the submarine force, with several officers already beginning to serve.
Two lawsuits were filed last year challenging the Pentagon's ban on women serving in combat, adding pressure on officials to overturn the policy. And the services have been studying the issue and surveying their forces to determine how it may affect performance and morale.
The Joint Chiefs have been meeting regularly on the matter, and they unanimously agreed earlier this month to send the recommendation to Panetta.
A senior military official familiar with the discussions said the chiefs of the Army, Navy, Air Force, and Marine Corps laid out three main principles to guide them as they move through the process:
That they were obligated to maintain America's effective fighting force.
That they would set up a process that would give all service members, men and women alike, the best chance to succeed.
That they would preserve military readiness.
Part of the process, the official said, would allow time to get female service members in leadership and officer positions in some of the more difficult job classifications to help pave the way for female enlisted troops.
Women make up about 14 percent of the 1.4 million active military personnel. More than 280,000 women have been sent to Iraq, to Afghanistan, or to jobs in neighboring nations in support of the wars. Of the more than 6,600 who have been killed, 152 have been women.
The senior military official said the military chiefs must report back to Panetta with the initial implementation plans by May 15.