US Defence Secretary Leon Panetta is to remove a ban on women in the military serving in combat.
The move could open more than 230,000 combat roles to women. It overturns a 1994 rule prohibiting women from being assigned to small ground-combat units.
The decision is expected to be formally announced on Thursday.
Pentagon chiefs will be asked to report back to Mr Panetta by 15 May on their initial plans to implement the new policy.
But they would have until 2016 to argue for any specific posts that they think should remained closed to women.
The BBC reports 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.
Women comprise 14% of America's 1.4 million active military personnel.