During the 1980s, British Rail (BR) was interested in replacing its first-generation diesel multiple units, particularly in the use of railbuses to service its lightly used branch lines.

Several single- and two-car prototypes were constructed and evaluated, leading to an initial production batch by British Leyland, designated Class 141 units.

Great Western Railway retired its Class 143 fleet in December 2020, while Transport for Wales was granted an extension and ran its trains until 29 May 2021.

By the beginning of the 1980s, British Rail (BR) operated a large fleet of first generation DMUs, which had been constructed in prior decades to various designs.

[10] While formulating its long-term strategy for this sector of its operations, British Rail planners recognised that there would be considerable costs incurred by undertaking refurbishment programmes necessary for the continued use of these aging multiple units, particularly due to the necessity of handling and removing hazardous materials such as asbestos.

[7] In the concept stage, two separate approaches were devised, one involving a so-called railbus that prioritised the minimisation of both initial (procurement) and ongoing (maintenance & operational) costs, while the second was a more substantial DMU that could deliver superior performance than the existing fleet, particularly when it came to long-distance services.

[7] While the more ambitious latter requirement would ultimately lead to the development of the British Rail Class 150 and the wider Sprinter family of DMUs, BR officials recognised that a cheaper unit was desirable for service on the smaller branch lines that would not be unduly impacted by lower performance specs or a high density configuration.

[11] Initial testing with the Class 140 uncovered several issues, such as difficulty detecting the type via track circuits, this was reliably resolved by swapping the material of the brake blocks from a composite to iron.

[7] Unlike the Class 141, a microprocessor-based controller for the automatic transmission was used from the outset, allowing the reliability issues posed by defective relay logic and poor earthing present on the predecessor to be entirely avoided.

This uncommon arrangement has been attributed with resulting in the Class 141 units possessing a relatively rough ride, especially when traversing jointed track or points.

During the 2010s, the rolling-stock leasing company Porterbrook proposed an extensive refurbishment of both the Class 143 and 144 units with the purpose of satisfying the diverse needs of this requirement; it was noted that the envisioned modifications would necessitate a significant reduction in the number of seats available.

On 17 October 2004, Wessex Trains unit 143613, forming a service from Bristol Temple Meads to Weston-super-Mare with 143621, caught fire between the site of the former Flax Bourton railway station and Nailsea and Backwell.

[25] Vehicle number ranges were as follows:[1] Some units received names:[27] Alongside the operational preserved Class 143s, some have been acquired for non-operational use at heritage railways or for non-railway use.