In test cases of national importance, today's ruling at the Court of Appeal will decide whether "bedroom tax" regulations, introduced last April, are an "excessive and unfair burden" on people with disabilities, or a lawful and "integral aspect" of the Government's deficit reduction programme.
Backed by the Equality and Human Rights Commission, lawyers for five tenants are arguing that the regulations cannot be allowed to stand.
The Department for Work and Pensions rejects the "bedroom tax" tag and says the reality is a "spare room subsidy" has been removed from social sector tenants.
The change in regulations is expected to produce savings of £500 million a year, but opponents say they have had a "devastating" impact on many people and fail to reflect the actual needs of disabled people for extra space.
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