A staff morale crisis, poor training and computer failures have brought more delays to payments from the Child Support Agency, a report has found.
MPs have said the CSA could be replaced unless service improves
The government research also showed staff had put false information into its computer system to speed up claims.
It found some payments are not being made as cases have not been moved from the old computer system to the new one.
The CSA said its service had "progressively improved" since the report, but accepted problems remained.
The research, carried out for the Department of Work and Pensions, suggested most of its CSA staff were "overwhelmingly negative" about the new computer system and unhappy with the level of training they had received.
Staff told researchers that difficult cases were sometimes deleted and others stockpiled and never dealt with.
Some staff knowingly entered incorrect details into the system to get it to move cases on, while others devised ways of avoiding talking to clients over the phone.
Morale was found to be so low that the "vast majority" of administrative staff wanted to leave.
Because of the computer difficulties there are about 760,000 cases still waiting to be moved on to the new computer system, the CSA said.
That meant some benefits, such as the £10 per week child payment premium, were not going to hundreds of thousands of its clients.
BBC social affairs correspondent Kim Catcheside said: "The new computer system was supposed to remedy the CSA's reputation of inaccuracy and delay, but it is even more inaccurate and a huge backlog of unprocessed cases has already built up."
A DWP spokeswoman said: "We are not disputing comments made by staff in this survey, but it was carried out last year and things have progressively improved since then."
The CSA said it had taken steps to improve staff morale and to improve the computer system.
But a spokesman added: "We will not jeopardise cases where money is already flowing to children by moving them on to the new system until we are sure it is working properly."
In November it was announced that the agency's chief executive, Doug Smith, was resigning. He will leave the agency at the end of May.
MPs have said the CSA should be replaced unless it was able to improve its performance.