The term "unitary authority" was first used in the Redcliffe-Maud Report in 1969 in its current sense of a local government authority which combines the functions of a county council and a district council. Strictly speaking, the term does not necessarily mean a single level of local government within an area, because in some cases there are also parish councils in the same area.
Although the term was not applied to them, county boroughs between 1889 and 1974 were effectively unitary authorities, that is, single-tier administrative units. Before 1889, local government authorities had different powers and functions, but from medieval times some cities and towns had a high degree of autonomy as counties corporate. Some smaller settlements also enjoyed some degree of autonomy from regular administration as boroughs or liberties.
The Local Government Act 1972 created areas for local government where large towns and their rural hinterlands were administered together. The concept of unitary units was abandoned with a two-tier arrangement of county and district councils in all areas of England, except the Isles of Scilly where the small size and distance from the mainland made it impractical. In 1986 a broadly unitary system of local government was introduced in the six metropolitan counties and Greater London, where the upper-tier authorities were abolished and their functions were split between central government, the borough councils and joint boards.
A review in the 1990s was initiated to select non-metropolitan areas where new unitary authorities could be created. The resulting structural changes were implemented between 1995 and 1998. Bristol, Herefordshire, the Isle of Wight and Rutland were established as counties of a single district; the district councils of Berkshire became unitary; the counties of Avon, Humberside and Cleveland were broken up to create several unitary authorities; and a number of districts were split off from their associated counties. The changes caused the ceremonial counties to be defined separately, as they had been before 1974. The review caused 46 unitary authorities to be created.
A further review was initiated in 2007 and was enacted in 2009. The review established Cornwall and Northumberland as counties of a single district; established unitary authorities in County Durham, Shropshire and Wiltshire covering the part of the county that was not already split off in the 1990s review; and divided the remainder of Bedfordshire and Cheshire into two unitary authorities. The review caused nine unitary authorities to be created.