The Regent III was available with bodies from a number of manufacturers including Park Royal, Metro Cammell Weymann and Charles H. Roe.

London Transport acquired 76 AEC Regent III buses with Weymann lowbridge bodywork.

Due to their lower height of 13 feet 6 inches (4.11 m), about 1 foot (0.3 m) lower than standard double-decker buses, a significant number found their way to other countries especially in Europe and the USA where maximum vehicle height restrictions allowed them to operate.

The AEC Regent III RT (RT-type) was first built in 1939 and was designed for and by London Transport.

Some RT-type buses were built for operation outside London, such as for St Helens Transport.