Photograph portrait of Princess Mary, half length, turned to her left, in the uniform of the Auxiliary Territorial Service (ATS) decorated with ribbon bars. Signed and dated by the sitter

The ATS was the women's branch of the British Army during the Second World War. Princess Mary became Chief Controller in May 1941. Later that year in December, Parliament passed the National Service Act, which called up unmarried women between 20 and 30 years old to join one of the auxiliary services, including the ATS, Women's Royal Naval Service (WRNS), the Women's Auxiliary Air Force (WAAF), the Women's Transport Service, Women's Voluntary Service and the Women's Land Army. Married women were also later called up, though those who were pregnant or with small children were exempt.