The Child Support Agency (CSA) is in "meltdown", former welfare minister Frank Field has warned.
The CSA had been repeatedly criticised since it was set up
The CSA is "bolting down the hatches" by refusing to publish key information, he says in an open letter to Prime Minister Tony Blair.
Reforms of the agency in 2003 made a "poor service even worse", he said.
Works and Pensions Secretary David Blunkett said there was nothing in Mr Field's letter that ministers were not "painfully aware of".
He has promised a review published in October would be "root and branch".
Lord Hunt, the minister responsible for the CSA, said the agency was now dealing with one million extra child support cases, a workload which would test any organisation.
He denied that the agency's performance had declined under Labour.
"I don't recognise some of the figures that Frank's been using, but what I do acknowledge, no question about it, the CSA has had major problems.
"We've appointed a new management team, they came in, the new chief executive was appointed in early summer and he's going to be reporting to ministers over the next month or two and then we're going to be looking at that and come out with a strategy for putting the CSA right."
Mr Field, Labour MP for Birkenhead, was once told by Mr Blair to "think the unthinkable" in proposals for welfare system reform.
"Organisations in meltdown typically bolt down the hatches to the outside world by publishing less and less information of what they are doing," he said in the letter.
"The 2003 reforms, while costing the taxpayer £456m for the new IT alone, have added to the agency's general chaos and declining performance and made an intolerably poor service even worse."
The CSA, "in practically all respects" was performing worse than it did a year after the 1997 General Election, Mr Field added.
He points to a number of factors which, he says, prove the CSA is in meltdown, including:
More than £1bn in maintenance now being written off
The backlog of applicants is still increasing, rising 20% in the last six months
The amount of maintenance collected by the agency has fallen in real terms for the first time