The National Service (Armed Forces) Act 1939 (2 & 3 Geo. 6. c. 81) was enacted by the Parliament of the United Kingdom on 3 September 1939, the day the United Kingdom declared war on Germany, starting the Second World War. The Act, which superseded the Military Training Act 1939 (2 & 3 Geo. 6. c. 25) of May 1939, enforced full conscription on all male British subjects between 18 and 41 who were present in Great Britain, subject to certain exemptions, and lasting until the end of "the present emergency". By a royal declaration in January 1941, the term Great Britain was extended to include the Isle of Man.
After the war ended, in September 1945, the Act remained in force until 1948, when it was superseded by the National Service Act 1948 (11 & 12 Geo. 6. c. 64), which extended conscription to times when the country was not formally at war, thus covering the Cold War and the Malayan Emergency.