The move would open up hundreds of thousands of front-line positions and potentially elite commando jobs to women.
The groundbreaking move recommended by the Joint Chiefs of Staff overturns a 1994 rule banning women from being assigned to smaller ground combat units.
Mr Panetta’s decision gives the military services until January 2016 to seek special exceptions if they believe any positions must remain closed to women.
A senior military official said the services would develop plans for allowing women to seek the combat positions. Some jobs may open as soon as this year. Assessments for others, such as special operations forces, including US navy commandos and the army’s Delta Force, may take longer.
The official said the military chiefs must report back to Mr Panetta by 15 May.
The official announcement on Mr Panetta’s decision is not expected until tomorrow.
Mr Panetta’s move expands the Pentagon’s action nearly a year ago to open about 14,500 combat positions to women, nearly all of them in the Army.
This decision could open more than 230,000 jobs, many in army and marine infantry units, to women.
In recent years the necessities of war propelled women into jobs as medics, military police and intelligence officers who were sometimes attached – but not formally assigned – to units on the front lines.
Women comprise 14 per cent of the 1.4 million active US military personnel.