The class 21 was a short-lived type of four-axle diesel-electric locomotives built by North British between 1958 and 1960. The first 38 were delivered to the Eastern Region and numbered D6100 to D6137. They had 1,000 hp MAN engines built under license in Britain which quickly gained a reputation of being very unreliable, as opposed to the original engines built in Germany. Additionally, it was reported that the position of various components in the engine room made maintenance complicated.

Nevertheless, 20 more were ordered for use in Scotland and numbered D6138 to D6157. They had a 1,100 hp variant of the same engine and had the advantage that the facilities of their manufacturer were not far away. Also the first 38 were soon transferred to Scotland. To increase the reliability of their engines, 20 locomotives received a 1,350 hp Paxman Ventura between 1963 and 1967 and were now designated class 29. The unrebuilt locomotives were withdrawn in 1968, but even the rebuilt ones were gone by 1971. Later, several types of MaK locomotives used around the Channel Tunnel again became class 21.