Action for Blind People is to become a subsidiary of the RNIB in a move that will see the two organisations combine their services across England.
From April, Action for Blind People will be classed as an "associate charity" of the RNIB, using a governance model that the charities say is new to the voluntary sector.
The boards of both charities have agreed in principle to the partnership but details have yet to be thrashed out.
Under the plan, the RNIB will use Action for Blind People as its 'delivery agent' to provide regional services in England. It will withdraw from some services itself.
Lesley-Anne Alexander, chief executive of RNIB, said the idea for the associate charity model came from her time working in the social housing sector, where group and subsidiary arrangements are common.
"We've come up with an innovative way of working that means we can build on the strength of both of our respective organisations while joining forces to be stronger and more effective," she said.
The new model would end "wasteful competition" and be less confusing for blind and partially sighted people, said Alexander.
Stephen Remington, chief executive of Action for Blind People, said the move was needed because services to blind and partially sighted people were "falling seriously short".
He said: "We've done joint work and had joint contracts in the past, so we've never been far away."
Action for Blind People had never contemplated a full-blown merger, he added. "We've been going for 150 years and you don't surrender your heritage and identity lightly."
Alexander said she hoped the model might be replicated in other parts of the charity sector.
"There are 700 charities for blind and partially sighted charities. I don't for a moment think there should be just one, but 700 is far too many."