The 4th Battalion was transferred to the Royal Engineers and converted, before the war, into the 50th (Northamptonshire Regiment) Anti-Aircraft Battalion, Royal Engineers. On the outbreak of war, it was serving with the 32nd (Midland) Anti-Aircraft Brigade, 2nd Anti-Aircraft Division. In August 1940, the battalion was transferred to the Royal Artillery and became 50th (Northamptonshire Regiment) Searchlight Regiment, Royal Artillery. Due to a severe shortage of manpower in the 21st Army Group fighting in North-west Europe, particularly in the infantry, the regiment was converted back into infantry, becoming 637th (Northamptonshire Regiment) Infantry Regiment, Royal Artillery and joined the 304th Infantry Brigade.