The new ruling means that such women are no longer able to claim family credit, which is usually worth around Pounds 46.50 a week, on top of their wages. To be eligible for family credit, a claimant or their partner must be in full-time work, defined as 16 or more hours a week. However, a social security commissioner has ruled that the hours of ancillary workers must be calculated over the full year, rather than just in term time. As most school ancillaries, such as dinner ladies and welfare assistants, are employed for under 20 hours a week, few work the hours needed to qualify for family credit under the new regulations.

Fresh guidance issued to benefit offices last week will be of little comfort to women who took up their posts before the ruling was made last June. The latest guidelines state that the ruling will not apply to claimants who have just started work. They will merely need to ask their employer to provide an estimate of the average number of hours they will work in a week, rather than a full-year average. However, when the benefit entitlement is reviewed - family credit claimants have to reapply every 26 weeks - the ruling will come into effect.

Some of the ancillary workers are caught in a new poverty trap; people working fewer than 16 hours are not entitled to income support, either. Many ancillary staff have lost their rights to free prescriptions and dental treatment and to a Pounds 40-a-week allowance for child care. The only workers to benefit will be those with an unemployed partner, who will then be able to claim income support.

“The people who will suffer most will be women on their own,” says Virginia Branney, a national negotiator with UNISON, the national and local government workers’ union. “They will again be caught in the legal entanglements of the social security system.”

The National Council for One-Parent Families has written to the DSS to protest. “We are particularly concerned about this because it is a change made by a commissioner, not by legislation, and we suspect that the minister didn’t even know about it,” a spokeswoman said.

The whole issue of state benefits and school staff is fraught with complications. There is a national agreement with school manual workers such as groundsmen and caretakers which says that their employment is “interrupted” during the school holidays, when they can claim unemployment benefit. But this does not apply to classroom assistants and administrative ancillaries, who are generally women.

To add to the muddle, decisions about eligibility for benefits are made locally, and so vary from area to area. Communications between DSS offices are poor, with different free phone helplines giving conflicting advice.

UNISON would like to challenge the new ruling, but to do so it would have to call for a judicial review, or prove that the commissioner had erred in law. The union is advising its members to seek the support of their local officers and to appeal against the ruling. One way out of the impasse could be for local education authorities to discontinue retainer pay over the holidays. Many ancillary workers would then be eligible for both unemployment benefit and family credit.

Meanwhile, a spokesman for the DSS said the situation was being monitored urgently to see if any change were needed.

Donald Dewar, Labour’s social security spokesman, said he had taken the issue up with Peter Lilley, the Social Security Secretary, but received “precious little comfort”.

“We will, however, continue to press him on this matter,” Mr Dewar said. “The 16-hours-averaged-over-the-year test is a nonsense. ”

Deborah’s story.

I was a wreck for about two weeks. I was too demoralised to put the Christmas cards up,” recalls Deborah Keeley, describing her reaction to the news that she was no longer eligible for family credit.

As a welfare assistant working 16 hours a week at Chestnut Lane first school, in Amersham, Buckinghamshire, she discovered that she would not qualify for income support either; you must work fewer than 16 hours to claim that benefit. After losing both entitlements, Mrs Keeley, who is a single parent, now had Pounds 65 a week to support herself and her three daughters, aged 12, 8 and 5.

“It seems crazy that it’s the hours that count, not the amount you earn, ” she says. “If your hours don’t fit, you can’t claim anything, even if you’re on the bread line.” To compound her problems, Mrs Keeley now had no right to free dental treatment and prescriptions. And it took the Housing Office so long to realise that she was no longer receiving family credit that they sent her a demand for Pounds 300 in back rent.

Mrs Keeley’s story has a happy ending; the school where she works has been able to give her six more hours’ work a week, bringing her up to the 22 hours she needs to get family credit. A sympathetic local DSS official has also advised her to appeal against the ruling. But the frustration at the arcane workings of the benefit maze remains.

“People like me want to work. We’re trying to avoid the stigma of income support, but it makes you wonder if it’s all worth it,” she says. “School work is ideal for single parents like me, yet they’re just making it difficult for us.”

Eliana’s story.

This ruling is totally penalising women. I don’t know many men working part-time in schools,” says Eliana Padghan, who works 17 hours a week as a welfare assistant in Chestnut Lane first school.

A single parent with an eight-year-old son, Ms Padghan was delighted when she thought she had worked out an employment package which would bring her off income support; she had also found a Saturday job in a shop. But when she telephoned a DSS helpline, she was told that school ancillary workers hardly ever qualify for family credit.

“I feel totally demoralised, and I can’t understand the official attitude, ” she said. “They save money if I’m no longer getting income support.”

Ms Padghan is now earning Pounds 15 a week less than she was on income support, and she has extra expenses now that she is working. “You can’t go in wearing holey jeans,” she points out. However, she is determined to struggle on as her school job is a vital step towards a new career; she is eventually hoping to train as a teacher.