The move could open hundreds of thousands of frontline positions and elite commando jobs to women. It overturns a 1994 rule prohibiting women from being assigned to small ground-combat units. But the military would have until 2016 to argue for any specific posts they think should remained closed to women.
The decision is expected to be formally announced on Thursday. The senior defence official told the BBC: "This policy change will initiate a process whereby the services will develop plans to implement this decision, which was made by the secretary of defence upon the recommendation of the Joint Chiefs of Staff."
Military chiefs will be asked to report back to Panetta by 15 May on their initial plans to implement the new policy.Some jobs are expected to be opened to women this year, while others - including for special forces such as the Navy Seals and the Delta Force - could take longer.This decision could open more than 230,000 combat roles to women, many in infantry units.
Senate armed services committee chairman Carl Levin welcomed the decision. "I support it," he said. "It reflects the reality of 21st-Century military operations." That was echoed by Major Mary Jennings Hegar, who served on three tours of Afghanistan with the US Air Force.
"It's not about whether or not we're allowing women into combat - women are in combat - it's about recognising their service," she told the BBC's Newsday programme. "I understand that stereotypically there are some physical differences but... there are a lot of men doing these jobs who are not built like Goliath. It's unfortunate that we have to hold women to a higher standard. "If you're able to meet the standards, then your gender should not hold you back. It's simple black and white discrimination."
But Elaine Donnelly from the US Centre for Military Readiness told Newsday that the move was "most unfortunate"."It's certainly not going to be helpful to our women in the military, or our men. It will cause enormous complications in the infantry battalions - the small tip-of-the-spear units, the ones that fight the enemy with deliberate offensive action under fire. These units are all-male for very good reasons. "The UK looked at this same issue some years ago and decided this was not a good idea, in 2002 and 2008. (BBC)