More than 50,000 families are now subject to the benefit cap since ministers introduced a lower limit on the amount of money they can claim, new figures show.
Department for Work and Pensions (DWP) data shows 68,000 households had their incomes capped under the policy as of May.
Of these, 52,000 have only been subjected to the cap since its threshold was reduced in November.
The figures also show that 71% of households capped have single parents, with 17,000 having a child aged two or under.
Earlier this year, the High Court ruled the benefit cap is unlawful and illegally discriminates against single parents with young children under the age of two.
Dalia Ben-Galim, director of policy at charity Gingerbread, said: "The High Court ruling on the benefit cap was unambiguous: single parent families with babies and children should be exempt from this policy.
"The figures today show that even greater numbers of the most vulnerable families are being affected by the cap for no good reason.
"It is not too late for the Government to withdraw its appeal. The current policy is driving families into poverty, rather than into work."
The Government has indicated it will appeal against the High Court's decision, with figures released to Parliament saying it could spend more than £100,000 on legal fees at the High Court and Court of Appeal.
DWP says lone parents can still receive benefits up to the equivalent salary of £25,000, or £29,000 in London under the cap.
Work and Pensions Secretary David Gauke also said the policy has been effective in helping people find work, with 34,000 households previously subject to the cap having returned to work since it was introduced in April 2013.
"It is right that people who are out of work are faced with the same choices as those who are in work and these figures show that the benefit cap has been a real success," Mr Gauke said.
"But behind these figures are thousands of people who are now better off in work and enjoying the benefits of a regular wage.
"With record levels of employment and over three quarters of a million vacancies at any one time, even more people have the opportunity to change their lives for the better."
The cap limits the total income households can receive from a series of benefits, such as jobseeker's allowance, housing benefit and child tax credits.
A single household can receive £20,000 in benefits, or £23,000 in London.
The DWP figures show 15,000 new families had their benefits capped in March, April and May, with 13,000 families moving off the cap over the same period.
The lower cap has also seen the proportion of capped households rise outside London.
Around half of those subject to the cap have their income reduced by £50 a week or less.
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