It has suggested that the trustees of local groups should consider the letter, make amendments if they wish, then sign it and send it back.
The letter reads: "We are excited by the opportunities the creation of a new charity creates for partnership working at a local level to benefit all older people. Working together we will all be better placed to face the challenges ahead."
The letter is part of the second phase of a wider consultation about the merger. Local ACs are independent and not obliged to agree.
A few are opposing the merger or objecting to its terms. Some are unwilling to lose the Age Concern name, which is owned by ACE; others want a stronger regional structure. The chair of one AC, who asked not be named, said: "There is considerable unhappiness with the process, and there may well be some Age Concerns, particularly the larger ones, that will not sign up with the new organisation."
A spokeswoman for ACE and Help the Aged said: "About a fifth of ACs have so far responded to the consultation. Most of those have indicated that they want to be part of the new charity, and only a very few, small ACs have said no.
"As most AC board meetings take place later this month or next, we are not expecting to have a full set of responses until November."
The planned merger - the third attempt in 10 years - was set in motion last year. Dianne Jeffrey, currently chair of the Anchor Trust, was recently appointed chair of the new organisation, and a £150,000-a-year chief executive is being sought. It will be the largest charity merger since Cancer Research UK was formed in 2002.
It is understood that Help the Aged trustees are anxious to see a solution to the question of local ACs before the launch of the new charity next April.
The membership of Heyday, the organisation for older people launched by Age Concern England two years ago, has fallen from 44,000 to 41,500 in the past year.
Heyday originally aimed to recruit 300,000 members, but has failed to perform and absorbed more than £16.5m of investment, £3.5m of it from ACE reserves.
A dispute about Heyday with Tony Page, former managing director of Age Concern Enterprises, led to his departure with an £815,000 payment.
Asked if Heyday would be part of the merged charity, a spokesman said: "This will be a matter for the new charity, but there are no plans to close Heyday."