The Government has been urged to scrap its controversial benefit cap after the High Court declared it unlawfully discriminates against lone parents with children under two.
Labour leader Jeremy Corbyn responded to the ruling by describing it as a "further demonstration of the failure of this government's austerity agenda".
After a judge in London found that "real misery is being caused to no good purpose", Mr Corbyn said: "The Prime Minister should accept the High Court's judgment and end this discrimination against parents and children."
Mr Justice Collins ruled in favour of four lone parent families in their action against the Work and Pensions Secretary over the benefit cap, which limits the income households receive in certain benefits.
He said the successful claim related to the "revised" benefit cap which "requires the parent in order to avoid the imposition of the cap to work at least 16 hours per week".
Lawyers for the families said that a reduced benefit cap, introduced last year, "drastically reduced housing benefits, leaving lone parent families across the country unable to afford basic life necessities to care for their children".
Mr Justice Collins announced that the regulations are "unlawful insofar as they apply to lone parents with a child or children under the age of two", as they involve "unjustified discrimination" against parents and children.
After giving his decision on Thursday he gave the Government permission to appeal.
A Department for Work and Pensions spokesman said: "We are disappointed with the decision and intend to appeal.
"Work is the best way to raise living standards, and many parents with young children are employed.
"The benefit cap incentivises work, even if it's part-time, as anyone eligible for working tax credits or the equivalent under Universal Credit, is exempt.
"Even with the cap, lone parents can still receive benefits up to the equivalent salary of £25,000, or £29,000 in London, and we have made Discretionary Housing Payments available to people who need extra help."
In his ruling, Mr Justice Collins announced: "Whether or not the defendant accepts my judgment, the evidence shows that the cap is capable of real damage to individuals such as the claimants.
"They are not workshy but find it, because of the care difficulties, impossible to comply with the work requirement.
"Most lone parents with children under two are not the sort of households the cap was intended to cover and, since they will depend on DHP (Discretionary Housing Payments), they will remain benefit households.
"Real misery is being caused to no good purpose."
Solicitor Rebekah Carrier, of law firm Hopkin Murray Beskine, who represented the families, said: "Single mothers like my clients have been forced into homelessness and reliance on food banks as a result of the benefit cap.
"Thousands of children have been forced into poverty, which has severe long-term effects on their health and well-being."
She added: "We are pleased that today's decision will relieve my clients - and other lone parent families around the country - from the unfair impacts of austerity measures which have prevented them from being able to provide basic necessities for their children."
The charity Shelter, which supported the challenge, said it was "calling on the Government to scrap the cap immediately, before it pushes even more people into homelessness".
The Child Poverty Action Group said it hoped it was the "beginning of the end for this nasty policy".
Equality and Human Rights Commission chief executive Rebecca Hilsenrath, said: "We need a welfare and support system that is fit for purpose and helps those in need.
"The vast majority of people want to work, but some, for legitimate reasons, are unable to do so.
"We have long argued the benefits cap is unfair on certain groups and has a particularly damaging effect on women and children from poorer families.
"We urge the Government to look again after this ruling."
Dalia Ben-Galim, director of policy at Gingerbread, a charity for single parents, said: "This is a fantastic result that offers real hope for some of the most vulnerable families in the UK.
"When it comes to single parents, the benefit cap rules risked pushing them into ever deeper poverty.
"We hope the Government will now commit to making sure this policy aligns with wider welfare rules for families with such young children."