History of local government districts in Middlesex

From the Wikipedia page

The History of local government districts in Middlesex outside the metropolitan area began in 1835 with the formation of poor law unions. This was followed by the creation of various forms of local government body to administer the rapidly growing towns of the area. By 1934 until its abolition in 1965, the entire county was divided into urban districts or municipal boroughs.

Poor law unions
The parishes of the county were grouped under the Poor Law Amendment Act 1834 to form unions. Each union had a single workhouse, and was administered by a board of guardians elected by the parish ratepayers. The boundaries of the unions would later be used to define rural sanitary districts in 1875 and rural districts in 1894. Poor law unions were abolished in 1930 by the Local Government Act 1929.