New figures from the Office of National Statistics published last week show an 80,000 fall in the Child Support Agency's (CSA) caseload since December 2008.
Last year, the requirement for single parents on benefits to make an application to the agency was dropped allowing them the choice about whether to use the agency or to make private arrangements.
Since November 2008 the CSA has operated as an arm of the Child Maintenance and Enforcement Commission (CMEC) and will be wound down in 2011.
Single parents' charity Gingerbread says it fears that the CSA's dismal past performance is deterring parents from applying to the agency's statutory service with the risk that they will settle for less than they need or end up doing without the money altogether.
Gingerbread wants to see far more active promotion of the CSA's services so that all single parents know they can still use the agency to recover any child maintenance owed to them.
Last week's figures show that the agency's performance is slowly improving with money getting through to 73% of cases where it's due. Overdue child maintenance payments were recovered for an extra 3,400 children in the last quarter.
Commenting, Gingerbread Chief Executive Fiona weir said: "The agency is seeing a drop in applications and we're worried that many single parents have lost faith in the statutory service and are turning their backs on the agency. Historically the CSA has not done well by parents with care of children and still owes parents £3.8 billion. But performance is slowly improving, and the CSA will be a better bet than private arrangements for some parents who cannot rely on child maintenance coming from the non-resident parent voluntarily.
"We are calling on CMEC to focus its communications on the poorest parents to make sure they know that the CSA can help get the money that will make a difference to their families. Without this effort, we fear the poorest single parents on benefits may end up doing without the money they need to put food on the table."