Up to 500,000 disabled people, their carers and families could lose out under the government's new Universal Credit system, due to be implemented across the country from October 2013.
A report by The Children's Society, Citizens Advice and Disability Rights UK states that 100,000 households with children could have incomes reduced by up to £28 a week.
The charities want the government to rethink the policy.
The system will replace Jobseeker's allowance, tax credits, income support, employment and support allowance - formerly known as incapacity benefit - and housing benefits with a single payment.
Universal Credit will see the total amount of benefits a household can claim across all payments limited to £26,000.
The report says that a further 230,000 severely disabled people who do not have another adult to assist them will receive between £28 and £58 less in benefits every week, while 116,000 working disabled people could lose up to £40 per week of support.
The authors also believe that the impact of the cuts in support for disabled children could be "extremely severe" for families currently receiving the mid-rate "care component" of the Disability Living Allowance, a payment made for children who can be severely disabled but do not need overnight care.
The report gathers views from disabled people; one in ten feared they may no longer be able to afford their own homes, two-thirds believed they would need to cut down on food, and more than half said the changes would leave them in debt.
Some families feared that the new Universal Credit would lead to their children needing to be placed in to residential care with cuts in mobility allowances biting.
Baroness Tanni Grey-Thompson, the independent peer who shares the title of Britain's most successful Paralympian with Sarah Storey, said the report did not make "easy reading".
She told the BBC: "Under the new system it is going to be difficult for a number of disabled people. The government say people are protected but it's only for current benefit claimants.
"What we want to do is ask the government to think again. We are in a situation where the regulations of the Welfare Reform Bill are coming to us quite soon and we can make changes. I think we can improve the system to help disabled people lead better lives."
David Cameron was asked about these changes at Prime Minister's question time, and stated that overall disabiity benefits would rise from £1.35bn in 2011 to £1.45bn in 2015.
"Under the plans, no recipients will lose out unless their circumstances change and all current recipients are fully cash-protected by a transitional scheme," he told MPs.
"What we are doing, and this is a decision and a choice we have made, is for future recipients we are going to increase the amount we give to the most severely disabled children and there will be a new lower amount for less disabled people.
"That is a choice we are making. Increasing the overall amount of money, focusing on the most disabled - that I think shows the right values and the right approach."
The report summarised three separate reports whose authors surveyed 3,500 disabled people and their families.