A quarter of a million children will be pushed deeper into poverty by the coalition's overhaul of the benefits system, a charity claimed on Tuesday.
Save the Children warned that a "blind spot" in the Government's new Universal Credit meant 150,000 working single mothers could lose up to £68 a week, or £3,500 a year.
Work and Pensions Secretary Iain Duncan Smith insists the new streamlined system, replacing the current array of benefits and tax credits, will make work pay.
But Save the Children said an impact assessment by the Department for Work and Pensions showed that the Universal Credit, while leaving many families better off, would also result in lower entitlements for 1.1 million households with children.
They would include a single mother with three children who earns about £242 a week, or just above the minimum wage.
Save the Children said gross income after housing costs would fall from £370 to £302.
Chief executive Justin Forsyth said: "Universal Credit will help some families but mums working hard to stay above the breadline are its big blind spot.
"It's incredibly hard bringing up three kids on £370 a week. Losing almost a fifth of that will push many families over the edge.
"The Government must make sure mums who want to work keep more of their incomes and get more support with childcare.
"Otherwise we'll see fewer women in the workplace and more children growing up in poverty."
A spokesman for the Department for Work and Pensions said: "Save the Children are wrong to assert that lone parents will lose as a result of the introduction of Universal Credit - the truth is 600,000 lone parents will be better off under a system which will incentivise work and make work pay.
"This is in stark contrast to the broken system this Government inherited which only rewards lone parents who work 16 hours or more.
"Under Universal Credit, 80,000 more families, including lone parents, will be able to claim childcare support - no matter how few hours they work."