The BBC's Chris Hogg reports on the staff who have left the CSA

The Public Commercial Services Union blames poor pay, constant pressure on workers and low morale for the situation.

The CSA said that staff turnover was running at 15% and that it shared union concerns but it denied there was a crisis.

Union officials say official management figures reveal that between January and May, 1,203 staff left the agency.

Losses have been worst in the front-line offices where civil servants deal directly with parents.

Harriet Harman drew up new proposals for the agency

According to the union, most workers at the agency - set up in 1993 to settle maintenance for children of separated parents - earn between £7,000 and £11,000.

Joint General Secretary Barry Reamsbottom called on ministers to take urgent action to halt the "haemorrhaging" of staff.

"These shocking figures are a telling indictment of the rock bottom morale in the agency," he said.

"The constant pressure from senior managers, the ridiculous rules set up by the previous government, the understaffing and the low pay are driving hundreds of hard working civil servants out of the CSA."

Mr Reamsbottom said the staff losses will "cripple" plans unveiled by the government last month to reform the agency.

'We share concerns'

Former Social Security Secretary Harriet Harman said the proposals would replace complicated maintenance formulas with a simple fixed rate calculation to make sure that more fathers pay maintenance.

But the PCS said the agency would now have to spend money earmarked for implementing new proposals on recruitment and training.

A CSA spokesman denied union claims that annual staff losses were running at 27%.

The agency insists that staff turnover is 15% including transfers to other departments, retirements and temporary workers.