Forget all the talk about a 48-hour working week: most mothers in Britain put in at least 100 hours and form the "hardest-working profession in Europe", according to a report today.

Business leaders may scoff at the idea that doing the laundry, the school run and vacuuming constitutes "work", but most mothers would disagree, pointing out that such labour is often physical, relentless, goes unnoticed and is certainly unpaid. Many also combine it with part-time paid work.

Research among 1,035 mothers found that for nearly 40 per cent of them, the day did not end until 9pm on a regular basis, and a third slept fewer than six hours a night.

For those in London, seven hours a week was devoted to the school run alone.

The research comes as a book on the stresses of motherhood, Perfect Madness: Motherhood in the Age of Anxiety, says women are living lives of silent desperation, trying to be supermums by looking after the children, keeping the home spotless and securing places for their children at the right school.

Here is one possible contributor to the phenomenon of the phenomenally hard-working mother: the tax system now favours two people in a couple working rather than one.

It used to be the other way round. It used to be that a married woman with children working part-time (which is very widespread now) would be taxed at the top rate of income tax incurred by her husband. In those days when the top rate of income tax was 83 per cent, that meant it was pretty much a waste of effort for the wife of a wealthy man to take a job. It was also not very remunerative for those with husbands on any above-average income.

Then Nigel Lawson introduced separate taxation. This was supposed to provide equality of treatment for women. But did it in fact contribute to the way that women now work incredibly hard?

He gave each member of a married couple a separate personal tax allowance and in each case, the rate of tax would rise first to the standard rate and only later to the top rate. So if, say, a husband and wife both earned £30,000, their combined income of £60,000 would mostly be taxed at the standard rate and over £9,000 of it would not be taxed at all. Whereas, currently, if all of it were earned by the husband, half of it would be taxed at the top rate and the tax free personal allowance would only be about £4,500 (or whatever the personal alowance has now reached).

Whereas before, for given combined family income, the tax advantage lay with only one of the two going out to work. Now the tax advantage lies with both of them having a job.

It might be better, perhaps, to go for something which - from memory - David Willetts suggested some years ago: a transferable personal allowance. A woman (or man) would be able to say to the inland revenue, 'I prefer to stay at home to look after the children and the home. I hereby transfer my personal allowance to my husband (or wife)'.

That would not entirely remove the tax disadvantage suffered by one-earner couples, but it would reduce it. In the process, it would allow many women the opportunity - but certainly not any compulsion - to stay at home looking after children and home if that is what they would prefer.

(The research that revealed the 100-hour working week of most mothers was commissioned by Comfort, manufacturers of fabric conditioner, but unfortunately the Daily Telegraph article on this does not say which organisation carried out the research.)