In a test challenge with implications for up to 50,000 mothers, Alison Davies, 43, is claiming £15,000 for maintenance that she failed to receive because of to calculation errors by the agency.

The case, before Warrington County Court today, is thought to be the first time that a mother has sued the agency for lost income.

She is arguing that it was negligent in its mistakes over her payments, which left her looking after her children for much of the time without maintenance from her former husband.

The CSA is seeking to have the action struck out and argues that it owes her no duty of care or “misfeasance in public office”, as she claims.

Last week in a Court of Appeal case, a senior family judge condemned the “sloppy, untidy drafting” in the CSA rules.

Lord Justice Ward said that the muddle — which led to the appeal — had deprived up to 50,000 mothers of their maintenance payments.

Mrs Davies, of Crowton, Cheshire, whose daughters are aged 16, 12 and 11, said that her former husband had two sources of income: as a self- employed lorry driver and the rent from a car sales garage that he owned.

But for much of the time since 2001 the CSA had ruled that she was owed nothing, leaving her solely responsible for bringing up her children. The error did not come to light until this year.

Mrs Davies, who is training to be a legal executive, said: “It is really an appalling situation. I wrote many times to the agency to find out what they were basing the assessment on, but they refused to say, because the details are covered by the Data Protection Act.”

Her solicitor, Stephen Lawson, of Forshaws, a law firm in Frodsham, Cheshire, said: “This case raises profound implications for the agency. If they are held accountable for their errors, the bill to the taxpayer will be astronomical.”

He said the agency confirmed that income from Mr Davies was recorded on its system and made two written statements to this effect. But it had not included the income received from the garage in any of its assessments at any time.

He said that a substantial amount of arrears built up which should have been used over the past four years for the wellbeing of the children.

Mrs Davies said that she had not received any explanation about why the agency gave her misleading information or maintained its position for such so long. “It causes tremendous stress on a family — not just the lack of money but trying to deal with something like this, because you are telling the children, in effect, that their father is not telling the truth.”

In last week’s case, Lord Justice Ward called on the Government to review the way in which earnings were assessed. An estimated 28 per cent of payments received in 2003-04 were incorrect — £125 million of the £601 million collected.

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