Disabled people are taking their legal challenge to the Government's so-called "bedroom tax" to the Court of Appeal.
Appeal judges will tomorrow consider several test cases brought by two law firms said to illustrate the "devastating" impact of the regulations up and down the country in social housing.
The appeal, expected to last three days, is against a High Court decision in July last year upholding the legality of new housing benefit regulations critics say are unjustified and unlawfully discriminate against the weak and vulnerable in society.
The regulations, introduced last April, led to reductions in benefit payments to tenants assessed to be under-occupying their accommodation.
Under new ''size criteria'', tenants with one spare bedroom have had a payment reduction of 14% and those deemed to have two or more spare, a reduction of 25%.
The appeal judges will be asked to rule that disabled people should be entitled to full housing benefit "for the accommodation they actually need".
Human rights lawyers say charities, social landlords and advice agencies have all spoken out about the plight of people with disabiilities who have been affected by the measure.
The Department for Work and Pensions (DWP) rejects the ''bedroom tax'' tag and says the reality is that ''a spare room subsidy'' has been removed from social sector tenants.
It says local councils are being given discretionary housing payment funding so that they can help vulnerable residents with all the welfare housing reforms, including disabled people affected by the removal of the spare-room subsidy.
The department says reduction of rising housing benefit expenditure is a legitimate and ''integral aspect'' of the Government's deficit reduction programme, and the change in regulations is expected to produce savings of £500m a year.
A DWP spokeswoman said: "We remain confident that we have fulfilled our equality duties to disabled people with the policy.
"Reform of housing benefit in the social sector is essential, so the taxpayer does not pay for people's extra bedrooms.
"But we have given councils £190m of extra funding this year to help those who need it."
Ugo Hayter, a lawyer from legal firm Leigh Day, said: ''We are very confident that the Court of Appeal will see that the decision to implement this legislation by the Government was clearly discriminatory and will overturn last year's ruling by the High Court.
"It is a cruel and deeply disturbing benefit cut which hits the most vulnerable in society."
Anne McMurdie, from Public Law Solicitors, said: "This case is about fairness. It is about disabled people being paid housing benefit to meet the size and type of accommodation they need because of their disabilities and not being financially penalised because they are disabled."
She added: "Since the benefit change was implemented in April 2013 there has been a wealth of research and analysis making clear the serious adverse impact on disabled people."