 The borders around the Manchester area were particularly complicated, with narrow protrusions of the administrative county between the county boroughs – Lees urban district formed a detached part of the administrative county, between Oldham county borough and the West Riding of Yorkshire.By the census of 1971, the population of Lancashire and its county boroughs had reached 5,129,416, making it the most populous geographic county in the UK.The administrative county was also the most populous of its type outside London, with a population of 2,280,359 in 1961.