The Act applied to males aged 20 and 21 years old who were to be called up for six months full-time military training, and then transferred to the Reserve.

It was the United Kingdom's first act of peacetime conscription and was intended to be temporary in nature, continuing for three years unless an Order in Council declared it was no longer necessary.

On 27 April 1939, Leslie Hore-Belisha, Secretary of State for War, persuaded the cabinet of Neville Chamberlain to introduce a limited form of conscription in Great Britain, but not in Northern Ireland, as a result of the deteriorating international situation and the rise of Nazi Germany.

The intention was for the first intake to undergo six months of basic training before being discharged into an active reserve.

There was one registration under the Act, of the first cohort of liable males, on Saturday 3 June 1939, and call-up for these men followed.