A High Court challenge is being launched over public library closures.
The challenge will focus on the legality of proposals by Somerset and Gloucestershire county councils to cut the number of libraries in their areas.
A legal firm has sent a letter before action challenging the reliance of the two councils "on Big Society community-transfer initiatives".
Public Interest Lawyers (PIL) said it was calling for a judicial review on behalf of local library users.
Councillor Antonia Noble, Gloucestershire's cabinet member for libraries, said: "I have every confidence that the council has acted legally and we will, if necessary, defend our position in court."
A spokeswoman for Somerset County Council said the challenge was in its early stages as the authority had only received a copy of the letter on Tuesday afternoon.
PIL's challenge questions the ability of David Cameron's Big Society to deliver statutory services to the public.
It said the councils' reliance on this conflicts with their "clear statutory obligation to provide a comprehensive and efficient library service for everyone wanting to use it".
The firm's lawyers also argue that the councils did not consult properly with local people prior to making the decision to cut, or pay proper attention to the needs of vulnerable groups.
PIL said in a statement: "Gloucestershire County Council proposes to reduce the number of libraries with full opening hours from 38 to nine, and to cut the mobile library service for persons in rural areas entirely.
"Somerset County Council initially proposed to cut 20 of 34 libraries and to reduce mobile libraries from six services to two.
"They have since announced that the cuts will be reduced to one third of libraries, but without showing how this would be financed."
Gloucestershire council has said it had received formal "expressions of interest" from groups interested in running all of the libraries being offered for community transfer.
A spokesman for the Somerset council said reductions in opening hours across all libraries in the county would help achieve the planned £1.35m saving and library users would be asked to make "voluntary donations".
PIL solicitor Phil Shiner said: "Libraries are at the heart of communities up and down the country.
"Councils cannot pin their hopes on vague notions of the Big Society when they are required by Parliament to maintain a comprehensive and efficient library service for everyone in the county.
"That means everyone, including single mothers, the disabled, the elderly and those living in rural areas."
The application for judicial review is expected to be launched at the High Court in London within a few weeks.