The 50 locomotives, then numbered D400-D449, were built at EE's Newton-le-Willows Vulcan Foundry between September 1967 and November 1968. BR initially leased the fleet from English Electric and it was not until 1973 that BR bought the locomotives. Upon introduction there were initial problems with their engines resulting in poor availability leading to new cylinder heads being fitted. Between 1969 and 1970 track improvements permitted higher speeds so the locomotives were required to work double headed. Although all were wired for MU working only D400 and D401 had been fitted with front mounted MU jumper cables from new so the rest of the class then had them retro-fitted.

The entire WCML had been electrified by 1974 so the locomotives were re-deployed as had originally been planned. Fifteen were initially retained by the MR for secondary passenger duties in North Wales and the North West, the r emainder were allocated to the WR as part of the drive to replace their non-standard diesel hydraulics. The introduction of TOPS saw the locomotives designated class 50 and renumbered 50001-50050. Having had their idiosyncratic motive power removed from them the WR decided to impose some of their style by naming the class after British Warships, thus replacing the original WR Warship class.

By the mid 1970s the class was suffering from unreliability partially due to poor design and maintenance but also because of inadequate air circulation in the engine room. The specially introduced electronic systems were proving problematic. A major refurbishment of the class was therefore undertaken at Doncaster works between 1979 and 1983. During this refurbishment the headcode boxes were plated over, though number of the class had had this work done prior to refurbishment. Also during refurbishment the bodyside window closest to the number 1 end was replaced by a grille. However the bulk of the work concentrated on improving the engine room airflow which included modifications to the roof and removal of the sanding apparatus and complex electronics. This generally cured the common Class 50 ailments and they settled down to provide a reasonably reliable service. The introduction of High Speed Trains on Western services in the late 1970s saw the Class 50s again relegated to secondary duties and this is where the class becomes associated with the Southern. The "new Warship" class replaced the native (but under-powered for the task) class 33s, which themselves had replaced the original Warship class, on Waterloo to Exeter services. The bulk of the fleet was inherited by Network SouthEast (the rest went to Departmental sectors) who continued to run them on Waterloo-West of England and Paddington-Thames and Chiltern workings until replaced by Class 47/7s in the early 1990s - the 50s lasting on Waterloo services until spring 1992. Their latter days were again plagued by unreliability and major frame cracking.

Of little relevance to Southern workings but brief mention should be made of the bold experiment in 1987 of modifying 50049 Defiance to a dedicated freight locomotive. Railfreight had previously rejected the class on account of the its low tractive effort, high maintenance costs and poor reliability and these trials were an attempt to convince the Sector otherwise. Perhaps the choice of this particularly named loco was deliberate! In this modification Defiance gained re-geared bogies, was renumbered 50149 and repainted into Railfreight livery. The experiment was not a success due in part to the removal of the wheelslip protection and sanding gear needed for heavy freight work during the refurbishment! 500149 was converted back to a standard 50 early in 1989 and finished its days working for NSE.

The final use of 50s in regular service was on 24 May 1992 with 50007 and 50050. This pair plus 50033 were retained until early 1994 for railtour use. The 50s have proved popular with preservationists with a remarkable 22 surviving, though not all are operational and it is possible that some may never be restored. All are naturally in refurbished condition though 50044 has been cosmetically restored as D444.