A number of local Age Concerns, including some of the biggest in England, have held talks about setting up an alternative national body to Age UK.
They fear they will lose autonomy and funding if they sign Age UK's branding and trading agreements, and that local services for older people will suffer.
They have discussed establishing a new federal structure and have contacted a national company to discuss the possibility of providing similar services, such as insurance, to those offered by Age UK.
"If we join Age UK it could change the entire culture of Age Concerns and lead to a loss of local control," said one chief executive, who asked not to be named.
A second chief executive told Third Sector she feared local Age Concerns, which are independent charities, would, in effect, become franchises, and a third said statutory agencies in her area had indicated they would withdraw funding if there was any loss of local autonomy.
Age UK was formed by the merger of Age Concern and Help the Aged in April last year. It has sent draft trading and branding agreements to local organisations but final versions have been delayed.
The breakaway group is alarmed by some of the requirements in the draft, such as passing information about beneficiaries to the national body and ceding control of their websites.
A spokeswoman for the group said its members did not want to reveal their identities until their trustees had considered the final contracts with Age UK, but they were concerned about the amount of time this was taking.
Age Concerns were told they would receive trading agreements by the start of June, then of July. But they are still waiting.
Helena Herklots, services director at Age UK, said it was finalising agreements and she was not aware of plans by local Age Concerns to break away.
"Getting the agreements right has always been - and continues to be - a collaborative process that very much involves local Age Concerns, with the ultimate aim of achieving the most for people in later life," said Herklots.
So far, 37 of 332 local Age Concerns have agreed in principle to become 'brand partners'.