The number of complaints about the Child Support Agency has increased by more than 50% in the past year, its watchdog has said.
Thousands of single parents have been left short of money
Delays, poor communication and agency errors were blamed for the rise by Independent Case Examiner Jodi Berg.
Referrals to the examiner increased by 52% to 2,151 in 2003-04, compared with 1,419 the previous year, she said.
Most needed only minor action, such as an apology, but 86% of investigated complaints were upheld, she added.
The Independent Case Examiner (ICE) provides a complaints review and resolution service for Child Support Agency (CSA) clients who have exhausted the agency's internal complaints procedure.
Ms Berg is launching the ICE's annual report for 2003-04 in Whitehall on Thursday.
Of the year's 2,151 referrals, 925 were accepted for action, 370 (42%) were resolved by agreement and 436 (58%) were the subject of formal investigation.
The agency, which is part of the Department for Work and Pensions, introduced a new method of calculating child maintenance in March 2003, in a bid to simplify the process.
But problems with a multi-million pound computer system meant payments to many single parents were delayed.
Ms Berg said: "This year the majority of complaint referrals related to old scheme cases, where delay, poor communication and agency error continue to cause
people hardship and worry."
She said she had focused her attention on "service failures under the reformed child support system, to highlight areas which have the potential to generate complaints.
"I welcome the agency's response to recommendations made in the reporting year."
CSA chief executive Doug Smith said the agency would continue to make improvements to its service and "work to provide a professional, efficient and sensitive service" for all its clients.
Work and Pensions Minister Patricia Hollis said agency staff were working hard to improve the service.
"The high priority the agency has given to customer service, complaint handling and improving its enforcement process have rightly been singled out [in the report]," she said.
"These are an indication of the agency's determination to improve the service it provides to clients.
"I am pleased that the Independent Case Examiner has recognised increasing levels of success in calculating maintenance on the new system in record time."