Many lone parents rely on child support maintenance payments
Changes to the way single parents claim child maintenance have been introduced.
The reforms are intended to simplify the system and make sure more of the money from absent parents reaches their own children.
But the One Parent Families/Gingerbread charity said some of the poorest families may lose out.
It fears as many as one in four single parents might stop using the Child Support Agency altogether and miss out on money they are owed.
The charity says the changes mean single parents will now have to decide themselves whether to continue using the Child Support Agency, make their own private arrangements or do without child maintenance altogether.
Fiona Weir, chief executive of the charity, said: "We fear that many poor single parents on benefit will struggle to agree private child support arrangements and their children may end up doing without.
"This would be disastrous for the children affected and for the government's child poverty targets."
She added that there was an urgent need for more investment in services to help parents deal with the financial and emotional consequences of separation.
"Only one in three eligible children get child maintenance. We need to build a culture that sees it as socially unacceptable not to support children after separation and to make sure that the most vulnerable do not lose out," she added.
The Child Maintenance and Enforcement Commission said the changes will mean all parents will have the same options for making a child maintenance arrangement that best suits them, whether a private arrangement or through the statutory maintenance service.
"Nobody is being encouraged to opt out of the statutory service and it will remain available to all those who are unable to agree private maintenance arrangements," a spokesman for the commission said.
He added that the commission would make sure parents have access to information and support to help them decide which arrangement best suits their circumstances.