Many lone parents rely on child support maintenance payments
The Child Support Agency is handing its powers to a new body, after years of complaints over delays, errors and with £3.8bn still owed by absent parents.
The new Child Maintenance Enforcement Commission has already said while £2bn may be recoverable, the remaining sum will most likely never be paid.
The new body has more powers to try to enforce maintenance payments, including asking courts to withdraw passports.
It will also encourage parents to make their own financial arrangements.
From Saturday, the new commission takes on responsibility for ensuring non-resident parents pay what they owe.
Since it was established in 1993, the CSA has been attacked for not delivering on its mission.
It was thrown into disarray in 2003 after a new IT system created a huge backlog in its operations - the Commons public accounts committee called the CSA one of the "greatest public administration disasters of recent times".
In 2006 the then Work and Pensions Secretary John Hutton announced it would be scrapped as it had a backlog of 300,000 cases and little prospect of recovering some £3bn in maintenance payments.
The CSA collects £36m in maintenance payment arrears a month - an increase of 40% in a year. In total about 68% of money claimed by parents is successfully paid out.
The new body will have tougher enforcement powers which will mean its staff will not have to go to court to take action against parents who refuse to make payments.