The chief executives of more than 30 charities have written to ministers urging them not to restrict access to a disability benefit.
The rules around Personal Independence Payments, which help with the extra costs of a long-term health condition or disability, will change on Thursday.
The charities claim people will be left without vital financial support.
Ministers said they were forced to act after two court judgements made the benefit more generous.
The Disability Benefits Consortium (DBC) said about 160,000 recipients of Personal Independence Payments would be affected if the changes go ahead.
Consortium co-chairman Phil Reynolds said: "Across the DBC we have had our helpline and advice services inundated by calls about PIP since it was introduced.
"Instead of supporting disabled people, the benefits system seems increasingly rigged against them.
"The whole system needs urgent improvement, in order to accurately assess the support they need. Disabled people cannot afford to wait."
The consortium comprises charities including Parkinson's UK, The MS Society and Mind.
Charities representing mental health and learning disability groups argue that the changes fundamentally fail to acknowledge that the costs associated with those conditions are just as severe as for other impairments.
The government said the new rules would save £3.7bn by 2023.