Cuts that have come into force at the start of the new tax year could force a further 200,000 children into poverty.
The “pernicious” move, which limits Universal Credit to the first two children in a family, will leave some families £2,780 a year worse off, the Child Poverty Action Group said.
And it hit out at the exemption process for women who have a child as a result of rape, saying the form was a “grave breach of claimants’ privacy”.
CPAG and the Institute for Public Policy Research estimate that 850,000 working families with more than two children will be affected. Two thirds of these will be families with three children.
Families that can comfortably support a third child today could struggle tomorrow
It warned the cut, which applies to new claims only, will harm children in larger families – a group which is already at greater risk of slipping into poverty.
Chief executive Alison Garnham said: “This is a particularly pernicious cut because it suggests some children matter more than others. It’s also illogical because no parent has a crystal ball.