The heads of more than 30 charities have written to the disabilities minister stating that they are "deeply concerned" by proposed changes to a disability benefit.
Thirty two chief executives signed a letter from the Disability Benefits Consortium to minister Penny Mordaunt, claiming that the changes to Personal Independence Payment (PIP) - set to come into force on Thursday - will leave tens of thousands without vital financial support.
PIPs provide support for the extra costs faced by disabled people - but Ms Mordaunt has said she will take action to ensure the payments go only to the most needy, after a tribunal ruled claimants with psychological problems who cannot travel without help must be treated like those who are blind.
The letter, signed by the heads of Parkinson's UK and Scope, amongst others, says: "We, the undersigned, as national organisations representing disabled people, are deeply concerned by imminent changes to Personal Independence Payment (PIP) which will leave at least 160,000 disabled people and those with long-term conditions without vital financial support.
"The decision to amend PIP following Upper Tribunal judgments means that disabled people face losing £3.7 billion by 2022. We believe this undermines the principle of the PIP assessment and its ability to fairly provide financial support, regardless of impairment or condition."
It adds: "Those with mental health issues, learning disabilities and autism face just as severe barriers and costs as those with other impairments - these changes fail to acknowledge this.
"We are also worried that the full scale and impact of these changes will not be understood before they come into effect. The Impact Analysis acknowledges a 'significant risk that the numbers affected could be much higher' than currently estimated."
Phil Reynolds, co-chairman of the Disability Benefits Consortium and policy and campaigns adviser at Parkinson's UK, 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."
A DWP spokesman said: "We are committed to ensuring our welfare system is a strong safety net for those who need it. That's why we spend around £50 billion a year to support people with disabilities and health conditions.
"Recent legal judgments have interpreted the assessment criteria for PIP in ways that are different to what was originally intended, so these amendments will ensure PIP supports those who face the greatest costs associated with their disability."