A total of 1.6 million children are now affected by the two-child benefit cap as the Labour government faces pressure to lift it.
The Conservative government introduced the cap, which prevents parents from claiming child tax credit or Universal Credit for more than two children for those born after April 2017.
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It means families cannot claim about £3,200 a year per extra child, the Resolution Foundation has said.
Economist at the organisation Lalitha Try said there was little evidence that policy had achieved its aims of boosting employment, but there was “clear evidence” of financial loss for families.
“Unless the policy is abolished, the majority of children in large families will fall below the poverty line by the end of the parliament,” she added. “Any new child poverty strategy should find the funds to remove it.”
The latest government statistics show a total of 1.6 million children in April were affected by the cap.
Of those 1.6 million, 1.3 million were in households claiming Universal Credit, who are on a low or no income, and 270,000 were claiming child tax credit, which is available to everyone.
The number of households affected has risen from 71,000 in April 2017 to 450,000 this April.
Sir Keir Starmer had previously said the cap should be scrapped, but U-turned last year, saying he would not axe it if Labour won power due to the state of finances they have inherited.
He has faced pressure to scrap the cap from within his party, including former Labour prime minister Gordon Brown, who said it is “condemning children to poverty”.
Former Conservative home secretary Suella Braverman surprised many when she also came out in opposition of the policy.
Labour MP Kim Johnson is now pushing for Sir Keir to include scrapping the cap in next week’s King Speech.
Ms Johnson told Sky News: “My constituency of Liverpool Riverside is the most deprived in the country. Nearly half of the children in my constituency are now living in poverty.
“The families I represent just cannot wait any longer. Labour has a huge task to undo 14 years of Tory decay and will have to make difficult decisions about what to prioritise.
“But I would like to see lifting the two-child benefit cap a priority for the new government as the most cost effective and most impactful way to immediately alleviate child poverty in communities like mine across the country.
“This policy has broad brush support across the political spectrum and I can think of no issue that is more urgent.”
The Lib Dems, SNP and Plaid Cymru have also called for the cap to be abolished.
Households are paid £25.60 a week for the eldest or only child and £16.95 for the second child, but nothing for any more children.
Child tax credit is available to anybody, however if either parent earns more than £60,000 a year they have to pay a charge, and if either earns £80,000 or more the charge will be the same as child benefit so most do not claim it.