II of 1918 (two different designs) had become obsolete, and their barrels had mostly reached the end of their usable service life.

A successor was sought and work began on an all-new design that would result in the Ordnance BL 4.5 inch medium field gun, a long-range medium gun designed for counter-battery fire.

The gun was in use throughout the Second World War and it equipped a number of medium regiments, including half the Canadian ones.

[citation needed] For the sake of expediency, Mk I ordnance was designed to be mounted on the 1918 60-pounder carriage.

It was withdrawn from field service in 1945, relegated to training purposes and finally declared obsolete in 1959 with the 5.5 inch gun replacing it.