Gavin Williamson has announced that all combat roles are now open to women, including serving in special forces units such as the SAS.
He said for the first time the “armed forces will be determined by ability alone and not gender”.
A ban on women serving in close combat units in the British military was lifted in 2016.
The Royal Armoured Corps, which operates tanks, was the first ground close combat branch to open its doors to women in November 2016.
Since then, about 35 women have either served in, or been trained to join, the Royal Armoured Corps.
However, retired officer Col Richard Kemp said the new policy would “cost lives”.
“My experience is if you have a team of men, even with one or two women, it could lead to divisiveness.”
“I am not blaming women, but anything that undermines that teamwork will cost lives.”