Families who say the benefit cap has had a "devastating impact" on them will learn the outcome of their High Court challenge to its legality.
Their action centres on the way the cap applies to many family members who act as unpaid carers of disabled people.
Their lawyer, Caoilfhionn Gallagher, is arguing that its implementation is "unfair and unlawful".
Ms Callagher told a judge in London at a two-day hearing in October that the proceedings concerned the impact of the cap "upon certain carers and the disabled people for whom they care".
She said: "Many carers are exempt from the benefit cap, but some are not.
"This claim is brought by two families - in both, an adult relative is providing full-time care to their elderly and disabled grandmother.
"The carers are able to perform their caring roles only with the support of state benefits, covering their housing and living expenses, and both are in receipt of carers' allowance."
Included in the group of individuals who are capped are those who receive a carers' allowance.
Ms Gallagher told Mr Justice Collins: "The claimants submit that the benefit cap as implemented is unfair and unlawful because of its impact on non-exempted unpaid carers and those for whom they care."
The submission of the families was that the "failure" of Work and Pensions Secretary Iain Duncan Smith to exempt them from the cap "has a devastating impact upon them, and others in a similar position".
They also argue that the "justification" by Mr Duncan Smith - who is contesting the action - "fails to take into account that carers contribute huge savings to the public purse".
If such carers were unable to look after their disabled relatives then "alternative care would need to be arranged, at great cost to the state".
Ms Gallagher explained: "To qualify for a carers' allowance, the benefit claimant has to be providing full-time unpaid care - upwards of 35 hours a week - to a severely disabled person who receives disability living allowance (DLA).
"This means that anyone receiving carers' allowance is not available to work sufficient hours to avoid the cap whilst they receive it, because they must be providing unpaid care on a full-time basis."
The lawyer told the court that the Secretary of State had provided an exemption from the cap to those who receive DLA, but not to all of their carers.
Paid carers were exempt, as were those who look after children or a partner or spouse, but "any unpaid carer who provides care to another adult, such as a parent or grandparent, is caught by the cap".
Campaigners argue that "unwaged" carers save the Government at least £119 billion per year.
They condemn the care allowance as "a pittance" at £62.10 a week for a minimum 35 hours and say it is hard to qualify for.
The judicial review actions involve three claimants. The first two are Ashley Hurley and her grandmother Mary Jarrett, who live in Peckham, south-east London.
Ms Jarrett, whose lung cancer is in remission, suffers from a number of conditions, including emphysema and arthritis. She has difficulty breathing and poor mobility. Ms Hurley receives a carers' allowance for the care she gives her grandmother.
The third claimant is Lee Palmer, who provides regular care and support to his 81-year-old grandmother, described as being in "very poor health".
Mr Palmer, who lives in the London borough of Tower Hamlets, is said to often provide "considerably more" than 35 hours a week of care, travelling to his grandmother's home five or six times a week.
Ms Gallagher told the judge: "Both of these challenges have at their core the submission that it is unreasonable to apply a cap aimed at 'workless families' to persons recognised to be providing, at a considerable saving to the public purse, at least 35 hours of unpaid care per week to a severely disabled person."
Mr Duncan Smith, is defending the Government's stance.
Clive Sheldon QC, for Mr Duncan Smith, told the judge that the Housing Benefit Regulations were "not irrational".
He said Mr Duncan Smith "does not accept that he acted unlawfully in proposing the exemptions and exceptions that Parliament has considered appropriate and that are therefore specified in the legislation".
The cap had the "clear and reasonable policy aims of making the benefit system fairer for tax payers by increasing incentives to work for households in receipt of high level of benefits and helping to incentivise behaviours that reduce long-term welfare dependency and contributing to the reduction of the budget deficit".