The Government has launched an appeal against a ruling that its controversial benefit cap unlawfully discriminates against lone parents with children under two.
A High Court judge in London said earlier this year that ''real misery is being caused to no good purpose".
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''.
But, at the Court of Appeal on Tuesday, lawyers for the Work and Pensions Secretary urged three judges to overturn his decision.
Clive Sheldon QC, for the Government, told Sir Brian Leveson, Lord Justice McCombe and Sir Patrick Elias that the judge had made "a number of serious errors", adding: "But for those errors, the claim would have been dismissed."
Mr Justice Collins was "wrong and erred in law in determining on the evidence before him that the benefit cap unlawfully discriminated" against the claimants who brought the action.
Mr Sheldon submitted there was "no discriminatory impact and, in any event, any such impact would be justified".
Lawyers for the families are asking the appeal judges to dismiss the Government's challenge.
After Mr Justice Collins ruled in June, Labour leader Jeremy Corbyn described it as a "further demonstration of the failure of this government's austerity agenda'', and said: "The Prime Minister should accept the High Court's judgment and end this discrimination against parents and children.''
Lawyers for the claimants 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 said the regulations were ''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.
In his ruling, he said: "'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.''
A Department for Work and Pensions spokesman said after the ruling that it intended to appeal, adding: "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.''
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