The Court of Appeal will rule today on the legality of Government cutbacks in the benefits system.
Judges will decide whether or not regulations for the so-called "bedroom tax" unlawfully discriminate against the disabled.
They will also, in a separate case, declare whether or not the "benefit cap" violates human rights laws and the common law because of its impact on vulnerable families.
Both cases are being considered by Lord Dyson, Master of the Rolls, sitting with two other judges.
The bedroom tax regulations, introduced in April last year, have led to reductions in housing benefit payments to tenants in social housing assessed through controversial "size criteria" to be under-occupying their accommodation.
Under the 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%.
Campaigners say the regulations have had a "devastating" impact on many people by imposing an "excessive and unfair burden" and failing to reflect the actual needs of disabled people for extra space because of their health problems.
Backed by the Equality and Human Rights Commission, lawyers for five tenants are arguing in test cases of national importance that the regulations cannot be allowed to stand.
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.
DWP lawyers argue the Government's aim of reducing rising housing benefit expenditure is both lawful and legitimate, and an ''integral aspect'' of its deficit reduction programme. The change in regulations is expected to produce savings of £500 million a year.
The DWP 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.
In the benefit cap case, it is said the Government's flagship cutbacks policy is having a particularly harsh impact on women fleeing domestic violence, and on their children, threatening to trap them in abusive relationships.
The case is being brought by two lone parents forced into temporary accommodation in London, and one each of their children.
The DWP argues the cap "sets a fair limit to what people can expect to get from the welfare system - so that claimants cannot receive more than £500 a week, the average household earnings."
The cap forms part of Government reforms to reduce spending on welfare by £11 billion per year.