The Southern Railway had started down the route to steadily expanding electrification in the 1920s, but an alternative form of motive power was needed for non-electrified routes and for hauling goods trains. After using the Derby type 2 BR class 24 locomotives as an interim solution to replace steam engines the Southern region specified what it wanted from a medium powered diesel locomotive. More horsepower was one request, but as the Southern was widely electrified changing to electric heating for hauled trains was desirable, allowing for locomotive or multiple-unit haulage.
The elimination of the steam heating boiler allowed BRCW to offer the longer 8 cylinder 1,550bhp Sulzer LDA engine accommodated inside the basic bodyshell used for the type 2 locomotives, classes 26 & 27, allied to Crompton Parkinson electrical machinery. Hence the classes' 'Crompton' soubriquet. The 33s were ordered as part of the second trance of diesel locomotives, benefiting from the experience of the pilot scheme designs, including the elimination of the cab end doors. This alone allowed three similarly sized cab windows to be fitted, greatly improving the aesthetic appearance of the locomotives.
Allocated to the Southern throughout their British Rail careers the class 33s were always used on inter-regional passenger and goods services, reaching Doncaster with cement trains in the earliest years and through the 1980s working daily to West Wales, Crewe and Manchester as extensions of the Portsmouth - Cardiff cross-country services.
A small number of 33s are still registered with Network Rail for mainline duties, mostly now as railtour support locomotives, in addition to more than 20 examples serving on heritage railways.