Following the building of the first 10 'Peaks' (Class 44), the British Railways Board decided that it wanted more of these engines but with a slightly more powerful motor installed. 127 were ordered and were built at Crewe and Derby between 1960 and 1963. Under TOPS, they became the Class 45. They were allocated to the London Midland and Eastern Regions.

The class was split into two subclasses in the mid 1970s. The 77 members of 45/0 remained basic locomotives with steam train heating equipment, while 50 members of the class had electric train supply fitted at Derby, during the period 1973-75, and became the subclass 45/1. Some of the locomotives had communicating doors in the cab fronts and split codeboxes while the rest had no doors and central codeboxes. Later, solid central codeboxes were fitted and, from 1985, the boxes received sealed beam headlights. When built, they were painted BR Brunswick green with a grey line at solebar height and progressed to BR blue in 1967. Displaced by HSTs, the last member of the class was withdrawn in 1988 and no fewer than 12 have been preserved.