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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. On 1 April 1974, under the Local Government Act 1972, the administrative county was abolished, as were the county boroughs. The urbanised southern part largely became part of two metropolitan counties, Merseyside and Greater Manchester. The new county of Cumbria incorporates the Furness exclave.

What is the most populous geographic county in the UK?
Lancashire