Jeremy Hughes, chief executive of the Alzheimer's Society, accused the Government of an "empty promise".
He said: "People with dementia and their carers are being forced to pay a dementia tax of tens of thousands of pounds a year for essential care and support.
"To them and thousands of other vulnerable people, an agreement in principle to a cap on care costs is meaningless.
"This empty promise also ignores the separate problem of underfunding of care as a whole. Some people are struggling with no care and no assets - for them, a cap on costs is useless.
"Further bickering between politicians will be a catastrophic failure of political leadership as families continue to struggle to care for people with severe disabilities.
"It is time for all parties to commit to a real public discussion about how we pay for the costs of care and make the costs for families fair. We cannot afford to delay any longer."