The Supreme Court is to hear a case challenging the way councils in England can consider their finances when assessing people for social care.
A severely disabled man - known as KM - and four charities will say care should be the same wherever a person lives.
Cambridgeshire County Council awarded KM about £85,000 a year, for care that a social worker estimated at £157,000.
If the appeal is successful, councils will have to reconsider how they assess the needs of disabled people.
Following a ruling in 1997 by the House of Lords, councils in England can currently consider their resources when assessing the needs of disabled adults.
But lawyers representing the charities say a council's budget should have no impact on the assessment of need.
"This is potentially the biggest community care case for 15 years," said Alex Rook, of law firm Irwin Mitchell.
He said the charities were seeking to "end the inequity of the current situation and determine once and for all that care needs are care needs, regardless of the local authority in question.
"Each of the charities firmly believe that a person's individual needs are the same regardless of whether they live in Hackney or Harrogate."
The four charities pursuing the case are Sense, the National Autistic Society (NAS), the Royal National Institute of Blind People and Guide Dogs.
Simon Foster, head of legal services at Sense, said: "Too often councils do not focus on the care that someone actually needs but on their own available budget."
NAS chief executive Mark Lever added: "All disabled people have the right to lead as fulfilling and rewarding a life as possible.
"Whilst we appreciate the challenging economic context in which local authorities are operating, assessments must be fair and the system must recognise that an individual's needs will be the same, regardless of where they live."
KM and the charities will argue that the law has been misinterpreted and that each individual should be assessed in the first instance in terms of what care they need, rather than the local authority's financial position.
The health secretary has intervened in the case and the Supreme Court has allocated seven judges to hear the legal arguments, which will be made on Tuesday and Wednesday.