A meeting of Falkirk Council next week will be asked to agree to split the Trust's functions between various council departments.
This would mean that culture, arts, museums and parks become part of Place Services; libraries move to Corporate and Housing Services; and sport and leisure becoming part of Children’s Services.
In a letter to the council's chief executive Kenneth Lawrie Maureen Campbell said: "Over the last decade the Trust has transformed from a council service into an effective organisation which is market-led, customer focussed and attuned to the needs of our communities and partners.
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"This combination of services, culture, and sport deliver health and social outcomes that go far beyond their component parts.
"To break this up would create disfunction, unnecessary complexity and is likely to eliminate any potential cost savings."
The board is particularly dismayed at the prospect of separating library services from cultural services.
Ms Campbell said: "Libraries play an important role in the cultural landscape.
"While book borrowing is declining, libraries continue to confidently articulate the intrinsic value of reading, listening, and understanding.
"We have concern that the core service will be undermined in Corporate and Housing Services, transforming library buildings into outposts for
a range of support services to the detriment of their primary purpose."
Falkirk Council's chief executive, Kenneth Lawrie, had previously proposed the new structure for the Trust but promised to consult the current leadership on the best way forward.
However, despite the fact that they say it would be better if the entire Trust was to move into Place Services, Mr Lawrie says this will not be possible.
The move to bring the Trust back in-house has been strongly condemned by former board members, who say councillors have not yet explained how they will make up for the loss of £1 million in rates relief every year.
Ian Scott, Bob Tait and Alex McQuade also wrote to Mr Lawrie to express their concerns.
The proposals will be discussed at a meeting of Falkirk Council next Wednesday.