A national demonstration against child poverty is being planned
Tackling the low pay of women is the key to ending child poverty in the UK, according to the TUC.
A third of women in work earn less than £100 a week, but the figure falls to 14% for men, the TUC says.
The government has pledged to halve child poverty by 2010 and end it a decade later.
But the latest figures showed that the number of children living in poverty rose for a second year to 2.9 million, before housing costs, in 2006-2007.
The main measure of relative poverty used by the government is the number of people living in households with income 60% below the median household, with the poverty line adjusted for family size.
Research by the TUC, the End Child Poverty coalition and the Fawcett Society said that women's low pay had "huge implications" for their children's living standards.
POVERTY LINE (WEEKLY INCOME BEFORE HOUSING COSTS)
Couple, two children aged 5 and 14 - £346
Lone parent, two children aged 5 and 14 - £271
Childless couple - £226
Single individual - £151
Half of all the child poor lived in working households, the report said. Women in Britain were more likely to be poor than others in Europe from the moment they conceived.
The TUC said that mothers were being trapped in part-time, low-paid jobs. More than 75% of part-time workers were female.
The gender pay gap for full-time workers was 17.2%, with the average hourly wage for men at £14.98 compared with £12.40 for women.
"As 40% of households are now headed by single mothers, this has concerning implications for tackling child poverty," said TUC general secretary Brendan Barber.
"It is vital the government tackles low pay and takes action to stop discrimination against mothers now."
Minister for Women Harriet Harman said the government had taken action to help mothers stay in work and increase their income, including improved maternity pay and affordable childcare.
"The gender pay gap has narrowed in the last 10 years by 5%, with the minimum wage being a significant factor," she said.
"But the government is determined to do more. That is why we will have a strong Equality Bill which we will set out to Parliament later this month."
The Institute for Fiscal Studies has said that the government will have to reduce child poverty by 300,000 a year for the four years following 2006-07 to meet its target of halving child poverty by 2010-11.
The End Child Poverty coalition is holding a demonstration calling for more action on tackling child poverty in October.