When county councils were introduced in England in 1889 about 20 % of the area of Middlesex , along with a third of its population , was transferred to the new County of London and the remainder became an administrative county governed by the Middlesex County Council that met regularly at the Middlesex Guildhall in Westminster , in the County of London .
When county councils were initially introduced in England in 1889 around 20 % of the area of Middlesex , and a third of its population , was transferred to the County of London .