Tom MacInnes, co-author of the report, said the fact child poverty fell overall but there was an increase in child poverty among working households is "almost certainly" a result of rises in child benefit and child tax credit payments in 2008.
He said more attention must be paid to getting working families above the poverty line.
"With more than half of all children in poverty belonging to working families, it is simply not possible to base anti-poverty policies on the idea that work alone is a route out of poverty," he said.
"Child poverty in working households must be given the same focus as out-of-work poverty. Until this happens, debates about poverty will continue to be misleading."
Alison Garnham, chief executive of the Child Poverty Action Group, said it "can’t be right" that in modern Britain children in working households live in poverty.
"We cannot hope to end child poverty when more and more children whose parents are in work find their lives damaged by poverty regardless.
"The recession has left it much harder to find full-time jobs and Britain’s economy is far too reliant on poverty pay.
"Working incentives have been damaged by the coalition’s cuts to tax credits and childcare.
"If successful, the universal credit may improve work incentives, but it is still years away so urgent action is needed by George Osborne in the new year to make sure having a job is a guarantee that you have enough income to raise a family on."