At least 25 hospital trusts, including those in the Thames Valley and Hampshire, have drawn up these age-based criteria, branded "cruel" by fertility experts.

In some cases, doctors offer treatment to patients only over the age of 36 - and in some cases not until they are 39. By this time, the fertility of an average woman will usually have declined by at least half.

Health officials have defended the criteria by claiming that younger women have more options available, such as adoption, and more time to try to overcome any unexplained fertility problems naturally. But experts say that the selection process penalises those women who may have had a chance of having children through IVF in their 20s.

Even hospital trusts that do not select patients on the basis of age are warning couples they may have to wait years to get treatment on the National Health Service. This is another blow for women who have tried for children for a long time, only to opt for IVF in their late 30s.

In Cumbria, some healthcare trusts, including Carlisle, Eden Valley and West Cumbria, have a three-and-a-half- year waiting list. Although it is offering couples two free cycles of fertility treatment, North Cumbria is turning down those who have children from previous relationships.

Earlier this month, women who put off having children until late in life were warned that they were defying nature and reducing their chances of conceiving. Doctors writing in the British Medical Journal criticised those women who wait until their late 30s before trying to have a baby.

However, the charity Infertility Network UK said the NHS was denying women the opportunity to become mothers because of its age-selection policies.

"People have waited, expecting to get treatment, and are then told they are not going to receive it, which is cruel and unfair," said a spokeswoman. "If you are in your 20s you have more time on your side."

Six months ago, the Department of Health issued new guidelines stating that trusts should be offering women at least one free cycle of IVF treatment. The National Institute of Clinical Excellence (Nice) also recommends that women aged between 23 and 39 should be eligible for free treatment.

But Thames Valley Strategic Health Authority confirmed that, in its area, eligible patients had to be between 36 and 40 at the time of treatment. And in Hampshire, the policy is to offer treatment only in exceptional circumstances: if, say, a woman has been unable to conceive after a serious accident.

Nice will hold a fringe meeting at the Labour Party conference in Brighton this week, at which health experts will highlight problems surrounding the provision of fertility treatment.

Dr Lawrence Shaw from the British Fertility Society called for fertility treatment to be available as freely as treatment for cancer. He is leaving the NHS, he says, because the Government is not giving enough priority to helping people who cannot conceive. "If someone has to wait until 35, then their fertility is not going to be as good as at 30," he said. "The problem is that the NHS is rationing services."