By 2001, this had risen to 67 per cent. One of the results is a boom in childcare, with the number of private nursery places in Britain quadrupling in the past decade.
Governments have become increasingly interested in the issue of who is looking after children pre-school and outside school hours. In the United States, huge amounts of government money goes on various payments, including welfare, to mothers who stay at home to look after their children. Perhaps, the reasoning goes, if we get mothers back to work - so paying taxes rather than claiming benefits - it would stem a haemorrhage of state resources.
Evidence from US research also shows that middle-class children get more pre-school childcare outside the home because their mothers can afford it and go back to work sooner than their working-class counterparts. These children also seem better prepared for the early school environment; they are more socially skilled and advanced in certain cognitive areas. But the research often glosses over the issue of the quality of the care.
You need a lot of adults to a small number of children, and the adults have to be around long-term so the children develop a healthy relationship with them. Poor quality childcare is not good for your child.
Good quality or not, measurement of hormone levels shows that those in childcare are more stressed than children at home. It is not necessarily a bad thing, but, certainly, this finding means childcare has to be carefully and thoughtfully constructed so it helps rather than harms children.
These complex issues aside, Number 10 has become convinced universal childcare will secure working women's votes in the next election: the idea is that childcare can be represented as a female freedom issue. Prime Minister Tony Blair labelled it the "new frontier for the welfare state", and he used his first speech since the summer break to pledge a 10-year "road map" for childcare, with, eventually, universal availability for under-fives.
Then Education Secretary Charles Clarke set out plans for his "educare", a one-stop service for parents, offering school and social care for children for 10 hours a day, round the year. You could interpret this as saying children should spend less time with their parents, and more with paid professionals, so parents can work more.
But is this in our children's best interests? Can we be confident the people they are left with are up to the job? Recent psychiatric surveys have found evidence of increasing teenage mental health problems, many of which seem related to family fragmentation and insecurity. Can we be sure we aren't storing up problems for the future by seeing less and less of our children?
There have also been accusations that the Government's push for more day nursery provision is concerned more with an economic than a child welfare agenda. The money parents pay in taxes as they return to work could not possibly fund the kind of quality childcare needed.
Psychiatric research over the years has established that continuity of care is vital for children. It doesn't matter that much who looks after them, as long as it is a warm, stimulating, attentive and, above all, constant presence. High staff turnover in cheap childcare facilities is a real threat to a positive childcare experience. Some even argue that children under three are best looked after on a one-to-one basis. But that would prove so costly that you may as well give mothers two years' paid maternity leave.
Professor Raj Persaud is a consultant psychiatrist at the Maudsley hospital and senior lecturer at the Institute of Psychiatry in London. He is this year's visiting Gresham professor for public understanding of psychiatry and will give a free public lecture at Barnard's Inn Hall, London EC1 on November 29. See www.gresham.ac.uk for more details. Email: firstname.lastname@example.org