From early times the City of London and the rest of Middlesex had different sheriffs and other administrative arrangements. However apart from this the historic county was not divided, for administrative purposes, until 1889. In that year the urbanised east of the county became part of the new administrative county of London; administered by the London County Council. The less built up north and west of the county formed the administrative county of Middlesex; administered by Middlesex County Council.
In 1965 the Greater London Council was formed. Greater London absorbed the administrative county of London and surrounding territory, including almost all of the administrative county of Middlesex. The south west part of the historic county (then the Staines Urban District and the Sunbury on Thames Urban District but from 1974 the borough of Spelthorne) was transferred to the administrative county of Surrey. A smaller part of the historic county, in the Potters Bar area, was transferred to the administrative county of Hertfordshire. In 1995 the village of Poyle, which had been a small area in the north of Spelthorne, was transferred to Berkshire.
The first part of this article covers the constituencies wholly or predominantly within the area of the historic county of Middlesex, both before and after the administrative changes of 1889 and 1965. The second part refers to constituencies mostly in another historic county, which included some territory from the historic county of Middlesex. The summaries section only refers to the constituencies included in the first section of the constituency list.
Article names are followed by (UK Parliament constituency). The constituencies which existed in 1707 were those previously represented in the Parliament of England.
In the distribution of seats in effect from 1974-1983 the official name of the Greater London seats included a London Borough prefix. In most cases this is not used in the article names. There were also instances when a distinctive constituency name had, at different periods, been prefixed by different borough names. In those casses the distinctive constituency name alone is usually used in the article name. See the notes column for further details.