Planning Aid for London (PAL) has informed volunteers in a letter that it will close its London office "because of major cuts to our funding".
The charity, which has 10 staff, said that Planning Aid Scotland (PAS) will, as of next month, take over management of the service for a year but has promised that the service for clients will remain unchanged.
The letter said: "The management committee is confident that this arrangement will enable PAL to continue to provide the effective service in London that you as volunteers have helped to build up over the last 40 years.
"We are keen to maintain a London 'face' for PAL which will be provided through our website and a new non-geographic phone number - so community groups and members of the public will hardly notice any difference in the service they already receive from us.
"PAL as a charity will continue and the trustees will manage the organisation, monitor the service provided by PAS and promote the service to people and organisations in London as well as continuing to seek funding and support."
Volunteers were asked in the letter to transfer their contact details to PAS, which will "progressively take over from the end of October with the handover being completed in November".
The letter said: "PAS will operate very much as PAL does now matching and allocating requests for advice by email, supporting volunteers if they need assistance and receiving case reports in respect of completed cases.
"They will administer and pay expenses claims."
PAS, PAL, Planning Aid England, which is run by the Royal Town Planning Institute, and Planning Aid Wales are all separate charities delivering similar services in their respective areas.
PAL started in 1973, according to its website, and was the first planning aid service before it spread to other areas.
It advises and supports individuals or groups to prepare development plans, make planning applications and appeal against decisions.
Its website says it also runs a "small affordable consultancy service, dealing with a range of planning and enforcement cases, for those who are not eligible for the free service". Its clients "are typically small businesses and householders", it adds.
The service has 10 in-house staff, including five qualified planners, according to its website, and over 100 volunteers.
PAL said it did not wish to comment further to the letter sent out.
More information on the service can be found here.