The London Defence Positions, more commonly referred to as the Mobilisation Centres, were the result of plans made in the 19th century. Prior to this, all the attention had been focused on stopping any would-be invader at the coast, with the notable exception of the Royal Military Canal, which served as both a means of transport and defence; it was also used as part of the GHQ system, seeing the construction of pillboxes along its length in 1940.

The 1859 Royal Commission on the Defence of the United Kingdom believed London to be practically indefensible in the event of any invasion (other than denying any would-be attacker direct access to the city by means of the Thames.) Several options were discussed and proposed, which in the initial stages included proposing a fort to be constructed on Shooters Hill in order to protect the Royal Arsenal at Woolwich. This was never realised. The final proposals instead called for basic earthworks for the use of Infantry and mobile artillery, which could be simply thrown up in the the event of war. In addition, they would be backed up with permanent defensive works which could act as stores, placed at intervals of five miles (8 km).

The London Defence Positions were built along a 70-mile (113 km) stretch from North Weald in Essex down to the Thames at Tilbury and then from the Darenth Valley in north Kent, stretching south and west along the line of the North Downs, with the final work being Henley Fort just to the south-west of Guildford in Surrey. Unlike those forts constructed for the coastal defence, these works did not follow an exact plan, resulting in varied designs being used. They were built to act primarily as munition dumps and stores, although they were also intended to be strong points along the continuous line of defences. During WW1, they were again used as part of a scheme which to all intents and purposes followed pretty much the original plan that the Victorians had devised (although this did include linking up with the existing fortifications built around the dockyard at Chatham.) From 1907 onwards, aspects of the line began to be sold off, the first being the general stores, located in barracks behind the actual line and in the inter-war period, a number of the forts themselves followed. Today nearly all are on private ground and only one (Fort Halstead in Kent) is retained for military-related use.

The fort at North Weald in Essex was reused in WW2, in part due to its proximity to the RAF fighter station and due to having an radio station built adjacent to the site; as such, it can be regarded as a part of the GHQ line.

Reigate Fort was purchased by the National Trust and having been restored, is open to the public.