The facts emerged during hearings last week of two Commons committees - work and pensions and public accounts - where the CSA's IT project was discussed.
The work and pensions committee questioned Doug Smith, chief executive of the CSA, on the agency's contingency plans.
"Our current judgement is that the system is being recovered successfully," he said. "EDS [the supplier] has shown the right level of commitment in terms of senior management commitment and proper resourcing. It is delivering the changes which are needed for successful recovery."
However, Smith, whose resignation was announced at the meeting, admitted there is "very limited option for contingency available to us. This system is EDS's. We effectively purchase a service from it. There are about half a million cases residing on it. There is no need to devote what would be significant time, effort and cost into fully exploring a contingency."
Despite the lack of a contingency plan, work and pensions secretary Alan Johnson said he had not decided whether to pull the plug on the system.
There had been an improvement but not enough to rule out the "nuclear option", Johnson said. "I have not made a decision yetÉ but it is right up at the top of the agenda," he said, adding that he would make a quick decision.
At the Public Accounts Committee hearing, John Oughton, head of the Office of Government Commerce, said the CSA system had passed two Gateway reviews - independent assessments of projects by the OGC.
But he said the reviews were "very early in the life of the Gateway process".
MPs heard that the CSA had received applications for support from 478,000 people over the past 18 months but of these only 61,000 absent parents have made any payments.
A major software release is due to be installed by 6 December which will allow hundreds of thousands of CSA cases to be migrated from the old to the new system. A further release is expected next year, which will allow "old" cases on the new system to be re-calculated under new, simplified rules.