The Auxillary Territorial Service (A.T.S) began life in 1938 when women between the ages of 18-43 were recruited to perform various tasks such as drivers, telephonists, despatch riders and engineers, presumably to take care of things back home while the men were away at war which was just around the corner.
These holdalls/kitbags would have been issued along with uniforms and other gear to go about their various duties.
This one was kindly donated by Ken MacTaggart who, along with many others, bought it from an ex-army outlet after the war when the Ministry of Defence were selling off excess gear.