In some cases, including notable legal contexts such as the Communications Act 2003, the M25 is used as a de facto alternative boundary for Greater London.

The road was built as planned despite some protests that included the section over the North Downs and around Epping Forest which required an extension of the Bell Common Tunnel.

Because of the public inquiries, several junctions merely served local roads where office and retail developments were built, attracting even more traffic onto the M25 than it was designed for.

[3] Policing of the road is carried out by an integrated group made up of the Metropolitan, Thames Valley, Essex, Kent, Hertfordshire and Surrey forces.

The crossing consists of twin two-lane tunnels and the four-lane QE2 (Queen Elizabeth II) bridge,[6][7] with a main span of 450 metres (1,480 ft).

[11] From here to junction 8, the M25 follows the edge of the North Downs close to several historic buildings such as Chevening, Titsey Place, Hever Castle and Chartwell.

The northern section of the M25 passes close to All Saints Pastoral Centre near London Colney, Waltham Abbey and Copped Hall.

[37] A report in 1981 showed that the M25 had the potential to attract office and retail development along its route, negating the proposed traffic improvements and making Central London a less desirable place to work.

New features included additional earth mounds, cuttings and fences that reduced noise, and over two million trees and shrubs to hide the view of the road.

[39] The southern section of what became the M25 through Surrey and Kent was first conceived to be an east–west road south of London to relieve the A25, and running parallel to it, with its eastern end following the route of what is now the M26.

In 1973, local residents had parked combine harvesters in Parliament Square in protest against the road, draped with large banners reading "Not Epping Likely".

[48] After a length inquiry process, chaired by George Dobry QC, the transport minister Kenneth Clarke announced the motorway would be built as proposed.

[50] The Prime Minister, Margaret Thatcher, officially opened the M25 on 29 October 1986, with a ceremony in the section between junctions 22 to 23 (London Colney and South Mimms).

[48][a] At the opening ceremony, Thatcher announced that 98 miles (158 km) had been constructed while the Conservative Party were in office, calling it "a splendid achievement for Britain".

[57] In June 1992, the Department for Transport (DfT) announced a proposal to widen the section close to Heathrow Airport to fourteen lanes by way of three additional link roads.

[64] Environmental groups objected to the decision to go ahead with a scheme to create the widest motorways in the UK, without holding a public inquiry.

[72] In 2006, the Highways Agency proposed widening 63 miles (101 km) of the M25 from six to eight lanes, between junctions 5 and 6, and 16 to 30, as part of a Design, Build, Finance and Operate (DBFO) project.

[23][77] In 2009, a £6.2 billion M25 DBFO private finance initiative contract[78] was awarded to Connect Plus to widen the sections between junctions 16 to 23 and 27 to 30, and maintain the M25 and the Dartford Crossing for a 30-year period.

[110][111][112] In 1996, Kenneth Noye murdered Stephen Cameron in a road rage incident while stopped at traffic lights on an M25 junction in Kent.

At the end of the 1980s, before the advent of automated speed enforcement devices, owners of supercars would meet at night at service stations such as South Mimms and conduct time trials.

[121] Times below 1 hour were achieved – an average speed of over 117 mph (188 km/h), which included coming to a halt at the Dartford Tunnel road user charge payment booths.

[127] On 29 October, two days before the 2021 United Nations Climate Change Conference, Insulate Britain protests blocked traffic from junctions 21 to 22 in Hertfordshire and 28 to 29 in Essex.

[128] The M25 is a form of social control to ensure disruptive elements in society are neutralised by keeping a significant proportion of them in continual motion.

The jams have inspired derogatory names, such as "Britain's Biggest Car Park"[130] and songs (e.g., Chris Rea's "The Road to Hell").

[16] The demon character, Crowley, had manipulated the design of the M25 to resemble a Satanic sigil, and tried to ensure it would anger as many people as possible to drive them off the path of good.

[133][134] The lengthy series of public inquiries for motorways throughout the 1970s, particularly the M25, influenced the opening of The Hitchhiker's Guide to the Galaxy, where the Earth is destroyed to make way for a hyperspace bypass.

[135] The M25 enjoyed a more positive reputation among ravers in the late 1980s, when this new orbital motorway became a popular route to the parties that took place around the outskirts of London.

[138][139] The message originally read "Peas", supposedly the tag of a London graffiti artist; the rest of the wording is reported to have referred to his frequent clashes with the law.

[142] A spokesman for Network Rail sympathised with the requests to restore the "much-loved graffiti", but said they do not condone people putting their lives at risk by trespassing.

This leads to "junction hoppers" who only use the motorway for a short distance before exiting;[146] their difference in speed when entering and leaving the main carriageway causes a domino effect, resulting in all vehicles slowing down.