The M25 from Dartford around the south side to Junction 15 with M4 near Heathrow lies within the South East Region, the other half being with Eastern Region.

Originally the motorway was not intended to be an orbital road, its function on the south side being to relieve the A25 through Surrey and Kent before joining M20 near Wrotham. When GLC’s proposed orbitals were abandoned following the Layfield Inquiry, the Minister of Transport announced that the M25 and the proposed M16 on the north side of London would become the M25 orbital motorway and would cross the Thames at Dartford. This resulted in a new scheme, Swanley to Sevenoaks, being added to M25 to complete the circle; and the renumbering of the motorway between Sevenoaks and Wrotham to become M26.

The road passes through some very beautiful countryside, for example along the foot of the North Downs and near (not "in" as was frequently claimed) the Darenth Valley in Kent and in the planning stages it was fiercely opposed by local residents. There were many Public Inquiries and some of them became extremely unpleasant, with the Department’s officers sometimes threatened and even pursued home at the end of a day’s proceedings. It was strongly argued by some that the road need not be more than dual 2-lane and even that the road was not needed at all. On the other side there was a strong lobby in favour of the road being made wider: but the feeling in the Department was that any attempt to go for more than the 3-lane carriageways could have resulted in it being lost completely. It is worth bearing this in mind when critics say that the road is too narrow.

When the South West quadrant of M25 was completed and opened to traffic the daily flows quickly rose beyond those expected and it quickly became clear that to the south of Heathrow the motorway would soon need to be widened. The two sections between Chertsey (A320) and Egham (A30) had been built with enough room adjacent to the central reserve to allow for an extra lane on each side and, along with junction improvements to give freer flows, work to install the additional lanes started in 1989.

A more detailed description of the M25 can be accessed by clicking here.