US Defence Secretary Leon Panetta has decided to lift the military's ban on women serving in combat, a senior Pentagon official has said.
The move could open hundreds of thousands of front-line 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.Lawsuit
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 defense upon the recommendation of the Joint Chiefs of Staff."
Military chiefs will be asked to report back to Mr Panetta by 15 May on their initial plans to implement the new policy.
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.