North London learning disability charity the Tamarisk Trust is to become part of the Self Unlimited Group. The trust, which has about 100 staff, will keep its name.
Although Self Unlimited, which employs more than 700 people, provides services for people with learning disabilities in eight English regions, it has had no presence in the capital.
Julie Riley, chief executive of the Tamarisk Trust, said joining forces with Self Unlimited would allow the smaller, "forward-thinking" charity to build its capacity.
Riley, who expects to continue in an executive role for the London region when the new governance structure is formalised at the end of November, added: "This is not a cost-cutting exercise, but an arrangement that will improve how we utilise our existing resources and, in due course, support an expansion of our service delivery."
Neither charity anticipates service cuts or job losses.