M25 motorway re-examined as a motorway route in Sir Charles Bressey's and Sir Edwin Lutyens' "The Highway Development Survey, 1937". Sir Patrick Abercrombie's "County of London Plan, 1943" and "Greater London Plan, 1944" proposed a series of five roads encircling the capital. The northern sections of the M25 follow a similar route to the World War II Outer London Defence Ring, a concentric series of tanks and pillboxes designed to slow down a potential German invasion of the capital. Little was done to progress these plans until the 1960s when the Greater London Council developed its London Ringways plan consisting of four
M25 motorway this time depicting these short sections named the route as the M16 but this changed before completion. The section from Potters Bar to the Dartford Tunnel was constructed between 1979 and 1982. Construction of the M25 continued in stages until its completion in 1986. Prime Minister Margaret Thatcher officially opened the M25 on 29 October 1986, with a ceremony in the section between J22 and J23 (London Colney and South Mimms). The initial tenders for the construction of the M25 totalled £631.9 million. This did not include compulsory purchase of land and subsequent upgrades and repairs. Soon after the motorway