No recent British government has put as many women offenders behind bars as New Labour. The most famous slogan that Tony Blair ever uttered in opposition – reputedly coined for him by Gordon Brown – was that Labour would be "tough on crime and tough on the causes on crime", seems only to be true of the first half of that dictum.

Between 1996 and 2005, the population of the women's prisons rose from 2,300 to 4,600. It has stayed close to that peak ever since. Currently, it is 4,460.

This is not simply because more women are getting into trouble with the law. It is also that the courts are much readier to hand out a prison sentence when there is a woman in the dock. In 1996, 10 per cent of women convicted of indictable offences went to jail. Within 10 years, the proportion had gone up to 15 per cent.

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There are, of course, some very dangerous and malicious women behind bars, such as Rosemary West, the Gloucester housewife, mother and multiple killer who – like the 1960s child killer Myra Hindley – will never leave prison alive. But most inmates of Britain's 17 women's prisons are serving short sentences for petty, non-violent crimes. A disturbingly large number are more of a danger to themselves than to anyone else.

The typical woman prisoner is a teenager or 20-something who has been caught shoplifting or dabbling in drugs, and will be out in a matter of weeks. More than a fifth – 21 per cent – are in prison for theft or handling stolen goods, and nearly two-thirds are serving sentences of six months or less. But this brief punishment can have traumatic consequences that last long after a prison term has been completed. About 120 babies are born each year in prison. There are mother-and-baby units in women's prisons for very young children but it is very rare for a prison governor to permit a child over 18 months to be raised behind bars. Unless there is a close relative ready to step in, the child will go into care, with little prospect of being reunited with its mother even when she is released.

"It didn't matter whether they were prostitutes, thieves or drug dealers – they never stopped being mums," said one ex-convict, mother of a boy, who asked not to be named. "Birthdays and Mother's Day were the worst. Uncontrollable sobs would echo through the corridors. No wonder so many women tried to kill themselves."

Lesley Butt, who has served three terms in adult prisons, is convinced that the constant turnover of young women is utterly wasteful. Her own experience brought her to the perhaps surprising conclusion that the most effective prison sentences are the long ones. It was her third sentence, of six years, that gave her the time to get off drugs.

"If you're going in on a short sentence, you haven't got enough time and support to sort yourself out. Then you come out and go straight back into crime, because when they put a woman in prison they take her whole life away. It's usually the woman who keeps a family together, so she has no family to go back to, and soon she's back." She added: "Prison should be for sex offenders, murderers and people who are violent. The others should be sentenced to serve in the community."

The statistics suggest that she is right. Almost two-thirds – 64 per cent – of the women released from prison during 2004 were in trouble with the law again within two years. The figures also suggest that women offenders need an effective inducement to get off drugs. An Oxford University survey found that of the 500 women convicts they sampled, in the six months before going to prison more than half had taken drugs every day. The same team found that 78 per cent were showing some sort of mental disturbance. That proportion would be 15 per cent in the female population as a whole.

After there were six deaths in quick succession inside Styal Prison, near Manchester, a Home Office commission, headed by Baroness Jean Corston, recommended that vulnerable offenders could be dealt with outside prison.

Its findings were welcomed by experts in the field – who then felt let down when little action followed. Juliet Lyon, the director of the Prison Reform Trust, says that the Government is aware of the wastefulness of keeping large numbers of women in prison, but that in the past "implementation has lagged woefully behind" the good intentions of ministers like Maria Eagle. But, she added, "with cross-party support, public opinion on her side and over 20 of the largest charitable funders offering to help, we hope that the minister secures the resources needed to deliver this policy at last".