A hundred thousand disabled children will lose out when a crucial welfare benefit is halved under controversial reforms.

Parents can now receive a maximum of £54 a week through tax credits to help with the extra cost of looking after a child with disabilities. But under the welfare reform bill, passed by the Commons last week, that benefit will form part of universal credit and be cut to £27 a week, plunging thousands of families below the poverty line, according to the Children's Society.

The government says the money saved will allow it to offer larger sums to children with severe disabilities. However, the Children's Society said some families would lose £1,400 a year. It is claimed this could cost families with a child born with a disability about £22,000 by the time the child reaches 16. The government says it will provide transitional payments to ensure people do not lose out, but the guarantee does not extend to new claimants and will not be protected from inflation. Cash protection will also be lost through as yet undefined changes in a family's circumstances.

Antony Best, 23, whose wife died of swine flu in January, leaving him responsible for three children under four, two of whom have disabilities, said the change would mean he would struggle to run the car he needs to take them to hospital. Best, from Bradford, said: "I receive £197 a month through credits and disability allowance for help with my eldest child, who has Down's syndrome, and I am applying for help with my youngest son, who has cerebral palsy. Without it I wouldn't be able to put anything in a fund for their futures and I don't know how I would run the car. It would have a huge impact."

Bob Reitemeier, chief executive of the Children's Society, called for the government to halt the reforms. "This cut threatens to push many disabled children back below the poverty line," he said "With 100,000 children affected by this, there are 100,000 reasons to rethink this policy."

The revelation follows angry exchanges in the Commons over welfare changes which are set to see 7,000 cancer victims lose their benefits. On Friday a Tory MP, Philip Davies, was condemned for suggesting disabled people should be allowed to work for less than the minimum wage to make them more attractive to employers.

The bill will hit hundreds of thousands of disabled people by cutting their housing benefit. Under the proposals, 670,000 social housing tenants with a "spare room" will lose an average £676 a year because their homes will be deemed too large for their needs. Two-thirds of those who will be affected – about 450,000 people – are disabled according to the government's impact assessment. Up to 200,000 of those receive disability living allowance and about 100,000 live in homes adapted to their needs.

The proposed housing benefit cuts mean many tenants will go into debt and others will have to move, says the National Housing Federation's chief executive, David Orr: "The cuts to housing benefit are extremely harsh. Under-occupation in the social housing sector should be tackled, but slashing people's housing benefit and pushing them into poverty is not the answer."

Richard Hawkes, chief executive of disability charity Scope, said: "Cuts to child benefit, disability living allowance reforms and the impact of local authority budget cuts are all having a cumulative effect that could risk pushing thousands more families into poverty as a result."

The Department for Work and Pensions said: "Our reforms don't necessarily mean that people will need to move and our discretionary housing payment fund will provide a safety net for those who need it, with £130m invested over four years to smooth the transition. We will ensure there are no cash losers when people are moved to universal credit. We have increased the number of children eligible for the higher rate of disability support, and the introduction of universal credit will lift a million people, including 350,000 children, out of poverty."