The owners of four care homes in Pembrokeshire are going to court in a bid to force the county council to increase the fees it pays them.
They will tell the High Court in Cardiff they will have to close as the income does not cover their costs.
The council said it paid a similar level to many in Wales and more than others, and was confident it would win.
South Pembrokeshire MP Simon Hart said he hoped the "test case" would provide a solution for 86 residents affected.
The High Court will be asked to decide whether the council should immediately start paying more money while the longer term dispute is resolved.
Care Forum Wales, which represents independent care providers, will provide written evidence on their behalf.
Senior policy advisor Barry Latham said: "These cases appear to be symptomatic of wider problem in the social care sector which is causing a postcode lottery in relation to the provision of care in Wales.
For further information visit: http://www.bbc.co.uk/news/uk-wales-south-west-wales-11712480