The British Imperial system of units was established on June 17, 1824, by the Weights and Measures Act. Strictly speaking, it is not correct to use the term British Imperial for units in use before that date, and they are better described as traditional British. In this system, the standard yard had a length of 0.914398416 m, making the league equal to 4828.02363648 m. (Cardarelli gives a value of 0.91443992 m, but this is in error.) For the detailed history of this yard, see Yard (British Imperial).
In 1963, a new Weights and Measures Act defined the yard in terms of the metric units, so that the value has since then agreed with the International yard of 0.9144 m, slightly larger than the earlier value (but smaller than the value current in the United States prior to unification), making the league equal exactly to 4828.032 m, conforming to an agreement with the United States.