Ann Chant, 51, credited with turning the agency round after its disastrous launch three years ago, is to head the Opportunity 2000 campaign to increase the number of women in senior management. Her predecessor, Ros Hepplewhite, resigned after 18 turbulent months in the job, apologising for "unacceptable standards", and saying she had had enough.
Liz Lynne, Liberal Democrat social-security spokeswoman, said: "In such a short period of time, to lose one chief executive is unfortunate, but to lose two is just careless." The Liberal Democrats have been uncompromising in opposing the CSA's "authoritarian" principle.
In a sign that Miss Chant's departure had taken the Government by surprise, the announcement said arrangements for appointing a successor would be made "shortly". Andrew Mitchell, the minister responsible for the agency, praised Miss Chant as "first rate", and Malcolm Wicks for Labour, which has toned down criticism of the agency recently, said: "Things were beginning to move in the right direction ... She provided strong leadership to the CSA after an extremely poor start and I am therefore concerned about the leadership of the agency at this sensitive time."
Miss Chant said: "My time at the CSA has often been hectic and certainly challenging but I am pleased that I will be leaving my successor an agency which has a steadily improving performance and excellent staff and managers in place to ensure the good record continues."
The agency was launched to trace absent fathers and extract child maintenance payments from them according to an "automatic" scale which replaced individual court orders.
But last week MPs accused ministers and Miss Chant of continued complacency. An all-party committee said that serious mistakes continued to be made and that it was "not appropriate" for Miss Chant to try to minimise them.
The CSA's watchdog, the Ombudsman, Sir William Reid, also expressed "puzzlement" that the agency was not making better progress, given all the money poured into it.
In its early months the agency was plagued by errors and ran into fierce protest from absent fathers, some of whom were outraged to be faced with demands that they face up to their responsibilities, but many of whom had justified complaints about mistakes or rigidities in the formulae for calculating payments.
Miss Chant has overseen a dramatic reduction in the agency's error rate. But Frank Field, Labour chairman of the cross-party Commons social security committee, has criticised the agency for effectively granting an amnesty to about 300,000 people who have failed to reply to letters.