The government is being accused of failing to collect hundreds of millions of pounds of child maintenance arrears.
The Gingerbread group said almost £4 billion of unpaid money has built up over the 23 years of the Child Support Agency, which is being replaced by the Child Maintenance Service.
The single parent group said the average amount owed to more than a million families was over £2,000.
Gingerbread Chief Executive Fiona Weir said: "Britain's child maintenance system is contributing to a culture where too many parents think it's optional, rather than obligatory, to pay their child's maintenance.
"The accumulated level of CSA arrears is staggering and completely unacceptable. With analysis showing that one-in-five families are lifted out of poverty by child maintenance payments, this is vital money that parents, and their children, can't do without.
"It remains unclear what will happen to money still owed for children once the CSA closes for good. The government is keen to move on and have a fresh start, but there should be no fresh start for those who still owe child maintenance for children today.
"The Government cannot walk away from its obligation to collect the millions that are owed to Britain's 400,000 families with outstanding maintenance claims. The CMS is in danger of repeating the mistakes of the past unless it sends a clear signal that non-payment will not be tolerated and child maintenance debts must be paid."
A Department for Work and Pensions spokesman said: "We actively pursue those parents who are not meeting their financial responsibilities to their children and in 92% of cases, parents are paying the money owed.
"We also have a range of strong enforcement powers and can take deductions directly from earnings and bank accounts or prosecute anyone found not meeting their commitments."