Based on their experience in World War I, the British infantry asked some sort of artillery for close support. The initial plan was for special batteries of artillery, but the cost was prohibitive and the mortar was accepted instead.
Virtually every civilian reference publication states the 3-inch mortar had a caliber of 76.2mm (3 inches), however, the “3-inch” mortar had an actual bore of 3.189 inches (81mm). Few British or American manuals on the 3-inch provide its actual caliber, even on diagrams of the mortar annotated with all other dimensions. During World War II the British modified the 3-inch mortar to allow it to fire captured Italian 81mm mortar ammunition in North Africa (the modification required a change to the firing pin design as the Italian ignition cartridge and propellant charge were different).
The ML mortar is a conventional Stokes-type mortar which is muzzle-loaded and drop-fired. Experience in World War II showed that it did not have sufficient range when compared to the German 81 mm s.Gw.34 mortar and so a new barrel was designed. This became the Mark 2 mortar which remained in service until replaced by the L16A1 81mm mortar.