The disclosure that 400 complaints have been lodged with the Registrar - the biggest against any single body - comes the day after the National Audit Office criticised the CSA for getting more than half its maintenance assessments wrong.
Yesterday, Elizabeth France, the Data Protection Registrar, said that in order to assess maintenance payments, staff at the CSA had been revealing personal and financial material about one partner to the other, which under data protection legislation they should not.
Most of the complaints centred on the disclosure of personal and financial information about a second partner to the first. She said: "I recognise the particularly difficult balance which needs to be struck when sensitive personal information is being shared in order to ensure proper decisions have been taken. However my concern is that every disclosure of information by the CSA should be properly justified."
Yesterday, launching her annual report, she said that complaints against the CSA now made up 7 per cent of her office's work. But the bulk of the 3,000 complaints a year - about 32 per cent - remained incorrect credit references.
Much of the registrar's work includes ensuring government policy does not impinge on people's rights to privacy and Ms France said she was in consultation with the Home Office over its plans to get teachers, health administrators, and benefit and housing officials to inform on suspected illegal immigrants.