The Department for Work and Pensions annual estimates show the proportion affected, almost was unchanged from 2011-12 to 2013-14.
Work and Pensions Secretary Iain Duncan Smith said UK poverty levels were the "lowest since the mid-1980s" and showed government reforms were working.
Commenting on the findings, Enver Solomon, Director of Evidence and Impact at the National Children’s Bureau said: "The government has made it clear that it wants to give every child the best start in life so the fact that overall child poverty is no longer falling and that the number of children in poverty that have a mum or dad working has continued to rise should be a major concern to the government.
"Financial hardship blights children’s lives in so many ways, affecting how well they do at school, their health and their happiness. Tackling it must be a top priority for government which now needs to reconsider its approach to ensure that the legal commitment to end child poverty is met.
"A wide-ranging cross-government strategy is required which not only focuses on the multiple causes of poverty but also ensures the incomes of the poorest are protected in the same way as they are for pensioners. Ministers must focus less on how to measure poverty and act decisively to introduce robust mechanisms for ensuring that progress in the fight against child poverty is swift and permanent."
A child is defined as being in poverty when living in a household with an income below 60% of the UK's average. Average household income in 2013-14 remained unchanged from 2012-13, at £453 a week - making the poverty line £272 a week.
David Cameron's official spokeswoman said the Prime minister "remains committed to doing more work to eliminate child poverty and that is precisely why the government wants to look at having an approach that is focused more on tackling the root causes of poverty than treating the symptoms".