Another 200,000 children will be pushed below the official poverty line under changes to Universal Credit, according to a new study.
Limiting the payment to the first two children in a family will affect up to 850,000 families, said the Child Poverty Action Group and Institute for Public Policy Research.
The biggest group to be hit when the changes come into force on April 6 will be working families with three children, who will miss out on up to £2,870 a year, said the report.
Larger, non-working families already have their benefits limited so are not the main target of the changes, the two groups said.
Alison Garnham, chief executive of Child Poverty Action Group, 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. Families that can comfortably support a third child today could struggle tomorrow and have to claim Universal Credit because, sadly, health, jobs and relationships can fail.
"Our analysis shows another 200,000 children will be in poverty once Universal Credit is fully rolled out, directly because of this cut.
"Surely children should not have their life chances damaged because of the number of siblings they have."
A Department for Work and Pensions spokesman said: "This policy helps ensure that parents on benefits have to make the same choices as those supporting themselves solely through work.
"Current claimants won't see any reduction in their benefits as a result of this policy and we will continue to pay child benefit for all children in a household."