The Class 123 was a design of Diesel Multiple Unit built for British Rail in 1963. They were built by British Rail at Swindon Works, and designed like all Swindon units as inter-city sets. Ten of the four-car sets were built and introduced in 1963. The units bear a visible similarity to the British Rail Class 309, however there is no 'relation' here as the two types were built by different manufacturers for different markets.