It follows on from the A20 at Swanley, meeting the M25, and continuing on to Folkestone, providing a link to the Channel Tunnel and the ports at Dover.

The road starts at its junction with the M25 motorway and A20 road just east of Swanley, then continues south east across the River Darent, north of Farningham through the North Downs, past West Kingsdown and Wrotham to meet the M26.

It would be £1.5 million for the eastern section 5.25 miles (8.4 km), to open by the end of March 1960, built by Costain, designed by Scott Wilson.

On Wednesday 1 June 1960, 5 miles (8.0 km) was opened at Forstal (A229) by Sir Leslie Doubleday, chairman of Kent County Council.

[citation needed] Plans for a bypass of Maidstone had existed since the 1930s, originally as an all-purpose project, before being upgraded to motorway standard in the 1950s.

This left the motorway in two sections, with the 14-mile (23 km) gap running via the A20 – this was referred to locally as 'The Missing Link'.

[16] A Controlled motorway scheme was introduced in West Kent between junctions 4 and 7, with variable speed limits.

[17] In August 2016, part of a pedestrian footbridge connecting areas of Ryarsh divided by the motorway was brought down – initially suspected to be the result of an impact by a digger from nearby works to widen the southbound bridge at junction 4 being carried on a low-loader that was moving along the hard shoulder.

[18] In the incident, the southern section of the bridge – which rested on a plinth south of the motorway and the cantilevered northern section[citation needed] – was dislodged and fell onto the carriageway below, landing on the trailer of a passing HGV and being narrowly avoided by a motorcyclist who suffered broken ribs taking avoiding action.

The motorway reopened with the Highways Agency having declared that the northern part of the bridge was structurally intact.

However this section of the motorway was again closed on the weekend of 3 and 4 September 2016 for the demolition and clearance of the northern bridge element.

This closes that part of the motorway and uses the area as a lorry park until the ferries and/or Channel Tunnel are fully running again.

In July 2020, the government announced that it had bought a site beside junction 10A to build the Sevington customs clearance facility and lorry park.

[27][28] Data from driver location signs are used to provide distance in kilometres and carriageway identifier information.