Child maintenance blunders now take up more than half of all rulings against the welfare department after a deluge of complaints.
A damning report shows 313 of the 597 cases upheld last year by the Independent Case Examiner involved the scandal-hit regime.
By comparison, the entire working-age benefits system took up 105 upheld cases.
In one case a parent was sent “threatening letters” demanding more than £23,000 in child maintenance arrears.
The government then admitted it was wrong - and the parent was in fact due a £3,000 refund.
The parent was offered compensation of just £50, before it was upped to £150 after a further complaint.
Shadow Work and Pensions Secretary Jonathan Ashworth blasted the “blunders”.
He said: “To treat child maintenance as an afterthought is simply a disgrace when four million children are in poverty.”
He added: “It’s time Ministers got a grip of this shambles and guaranteed families, children and pensioners the security they deserve.”
The ICE, a watchdog that polices the Department for Work and Pensions (DWP), is seeing its highest number of overall complaints for a decade apart from the WASPI pensions campaign.
The number of complaints accepted for a review in 2021/22 - 1,708 - rocketed 68% compared to the year before.
There were 313 fully or partially upheld complaints about either the old Child Support Agency or the Child Maintenance Service, which replaced it in 2012.
Some historic Child Support Agency debts have been written off by the government but others are still being chased.
Unpaid Child Maintenance Service debts hit £493million in June and are rising by £1m a week. They are due to hit £1bn by 2031.
Single parents’ charity Gingerbread says the rate of enforcement is a “disgrace”.
But other parents have hit out at the CMS's approach to repayments, saying it has driven people to poverty and homelessness.
A DWP spokesperson said: “We support millions of people each year to get the help and service to which they are entitled.
“Only a very small number of complex cases go to the Independent Case Examiner, who provides a vital avenue of review.
“The Child Maintenance Service collected and arranged a record £1 billion for children of separated parents last year and is a significantly better service than its predecessor, managing over half a million arrangements for over 800,000 children.
"It helps lift 140,000 children out of poverty on average each year and we continue to improve it through automating simple processes, spending more time with vulnerable customers and strengthening powers to recoup maintenance from parents stubbornly refusing to pay what they owe."
