There were 3.9 million children living in “relative poverty” in 2014-15, up from 3.7 million a year earlier, the figures from the Department for Work and Pensions show.
It was the first increase, when housing costs were included, since 2011-12. An individual is considered to be in relative poverty if their household income is less than 60% of median income.
Two-thirds of children in poverty are living in households where at least one adult is in work.
Sam Royston, who chairs of the End Child Poverty Coalition, said the figures were “simply unacceptable”.