The study, from the Institute for Public Policy Research (IPPR) think tank, looked at the number of children living below the poverty line in families where someone works.
It found the number had remained static at 1.4m since Labour came to power in 1997. In contrast efforts to tackle child poverty in families where no one works have resulted in 600,000 children being raised above the poverty line.
Graeme Cooke, one of the authors of the Working out of Poverty report, said: "Government efforts to tackle child poverty have worked to some extent, but they have focused on those who are out of work.
"The risk of being poor is still much higher if you are not working. But the amount that you are better off sometimes isn't enough to take you above the poverty line."
The IPPR is calling for a personal tax credit allowance for the second working adult in couple families, an increase in the Working Tax Credit for couple families, and a higher minimum wage.
Kate Stanley, head of social policy at the think tank, said: "More action is needed to combine financial support and measures to boost parental employment with action to deliver fairness on pay and opportunities for progression at work."
