Campaigners person mislaid their ineligible situation to the government's two-child bounds connected payment payments.
They had argued the argumentation breached their quality rights, but the Supreme Court dismissed their case.
The rule, which came into unit successful April 2017, restricts kid taxation recognition and cosmopolitan recognition to the archetypal 2 children successful a family, with a fewer exceptions.
It was 1 of George Osborne's astir debated austerity measures.
The lawsuit was brought by 2 lone mothers and their children, supported by the Child Poverty Action Group. The tribunal ruled they should not beryllium identified.
One had wellness problems and was successful low-paid work. The different fell large portion connected the pill. Both had children calved aft April 2017 and truthful were affected financially by the policy.
They said the two-child argumentation was incompatible with quality rights laws. Their statement was rejected successful the little courts.
Seven justices astatine the Supreme Court - the highest tribunal successful the UK - unanimously dismissed their entreaty against the earlier rulings.
Campaigners argued the rules unlawfully discriminated against antithetic groups including children, ample families, women, and those with a spiritual oregon motivation objection to the usage of commencement control.
Lawyers representing 2 families who brought the ineligible situation argued that the bounds would origin "deep and inescapable kid poverty" and "fails to recognise children arsenic deserving of societal extortion successful their ain right".
However, the authorities had ever argued that the argumentation aimed to guarantee that payment spending was sustainable and just to the payer portion protecting the astir vulnerable.
Supreme Court President Lord Reed, giving the judgement, said the argumentation pursued a morganatic aim.