‘You cannot imagine how much women like me hate women like you.” This salvo was aimed at me at a drinks party 20 years ago by a successful publisher and well-known childless 1970s feminist, on noticing that I was pregnant.

“You think that just because you have lovely babies or terrible teenagers you have a God-given right to leave the office any time you like, to go to their nativity plays or their parents’ days, while the rest of us do your work for you and keep your lovely job warm for you, without any thanks, until you see fit to come back from your blissful maternity leave or your half-term holidays.” That was her drift.

I didn’t bother to tell her that I did not belong to the army of exploitative mummies she so loathed. I worked freelance and part-time from home and still do, precisely because of these problems. I was impressed by the force of her argument. She had worked hard all her life to make small enterprises succeed and, among the many risks involved, she particularly resented the risks of hiring women who turned out to be a liability and a source of disruption and resentment among her colleagues.

Nothing has changed since then. Working flexibly is good for families and good for mothers, but it is not good for employers. With the best of goodwill on all sides, flexitime will always cause problems. Some employers may be large enough to bear the inevitable costs and inconveniences. The public services will be protected from such commercial realities by the state and the taxpayer. But that does not change the unpleasant fact that flexible working imposes costs and inefficiencies on almost all employers and the economy as a whole.

Despite this obvious fact, more and more women now see flexible working as a human right. Men are beginning to do so, too. Politicians agree: both Gordon Brown and David Cameron are anxious to oblige. Downing Street announced last week an inquiry into extending the right to flexible working (currently restricted to parents of children under six) to parents of children under nine, 12 or even 17. About 6.25m parents have the right to request conditions such as flexitime or working at home; if all parents of schoolchildren were included, 4.5m more would have this right. This would include the right to time off to help teenagers with their exams.

Last month I found myself at a similar drinks party to the one 20 years ago, talking to another successful woman publisher and a woman lawyer, both of feminist views. The lawyer admitted sadly that in her small organisation she could scarcely afford to employ women, no matter how good, no matter how much better than the male applicants; if they disappeared for many months’ maternity leave with the right to return, it was almost impossible to replace them temporarily with a woman or man of the same calibre; why would any such high-flyer accept a temporary job for only a year or so without any security?

What would happen, meanwhile, to the discriminating clients and their complex affairs who had temporarily been abandoned? Would they want her back, pending the birth of her next baby? And this, the lawyer said, was quite aside from the direct financial costs of subsiding the mother’s pregnancy and flexitime.

The publisher agreed. If a good literary agent disappeared for a year’s maternity leave, she said, her firm didn’t bother dealing with the replacement. It took time and there were so many good agents around; her company would publish books from the other agents. So the literary agency of the woman on leave and the writers she looks after would lose out for at least a year.

The same problems emerge everywhere. In my own experience, women social workers and women doctors who work flexibly become much less satisfactory to me as a customer; they must be even more unsatisfactory to their employers. For instance, one excellent social worker I contact sometimes about one of her vulnerable clients works only on Thursdays and Fridays. If this client of hers suddenly has a big problem between Monday and Thursday morning, she won’t be available. Someone else may be, but it won’t be someone who knows and understands all the personal details. The costs of handovers between flexi-workers in complicated jobs like these must be astronomical, too.

I found the same with an all-women GP practice of excellent doctors. We gave up going because there was so much handover, so many temporary doctors and so little continuity – all because of maternity leave and flexible working. Our GP now is a man who works full time, as do his colleagues.

Some time ago I sat on a working group for the Royal College of Physicians, looking into the choices and opportunities for women hospital doctors. There was almost universal agreement that what women doctors wanted was the chance of flexible working and that many men doctors would like it, too. I sympathise; flexibility is what I’ve always needed as well. But one or two expert witnesses sounded cautionary notes.

What happens, one asked, in a small hospital where there were, say, three consultants in a speciality – two mothers and one man? Surely there would be great pressure on the man to accept more than his share of the family-unfriendly shifts. Two other witnesses, one a woman, suggested something more depressing. You could not get to the top of your profession, they argued, without making great sacrifices, including the sacrifice of time. Achievement in medicine – as in anything else – involves weighting the work-life balance heavily in favour of work. Flexitime is at odds with great achievement.

Anecdotes such as these are no substitute for argument, of course. But I mention them because they illustrate something which so many women (and men) seem determined to ignore. However good it sounds in theory, in the nasty detail of practice, flexible working all too often imposes a burden on businesses, on standards, on services, on clients and on the economy.

To impose flexible working on employers as a woman’s right and increasingly as a man’s right, too, is yet another step along the road of economic decline. In this light, resentment of flexi-workers doesn’t seem to me to be unreasonable.

minette.marrin@sunday-times.co.uk

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