Doctors have managed to extract eggs from five-year-old girls and freeze them for use when they are old enough to have children.

The scientific advance, which had been thought impossible, will enable girls suffering childhood cancers such as leukaemia to become parents in later life. Thousands are left infertile each year after undergoing chemotherapy.

It also opens the possibility of storing girls’ eggs to protect them against any form of infertility in later life.

Previously it was believed the eggs of prepubescent girls were too immature to be extracted. It was thought they became viable only at puberty by reacting to hormonal changes in the body.

Israeli doctors have, however, managed to extract the eggs and then culture them in test-tubes to make them viable. The resulting eggs are no different to those of a 20-year-old, say the doctors.

The doctors are part of the same research team that made medical history by treating a mother who gave birth to twins from embryos frozen 12 years earlier. The experiment demonstrated that eggs or embryos could be frozen for lengthy periods without damage.

Dr Ariel Revel, a lecturer in obstetrics and gynaecology at Hadassah University hospital, Jerusalem said: “We have developed a technique which allows us to obtain eggs even from girls and to mature them in vitro. We isolate eggs from the tissue and, following one or two days in culture, they are mature enough for freezing.

“The results are quite surprising. Since these girls are not menstruating, we would expect maturing them to be very difficult, but we have shown that they do mature. The cancer treatment these girls are having will almost certainly cause them to become infertile. Although this is still experimental, any technique that tries to restore fertility to these girls is worth trying.”

Revel will present the full results of his research at the annual meeting of the European Society of Human Reproduction and Embryology in Lyon, France, this week.

About 1,500 children undergo treatment for cancer every year in Britain. Seventy-five per cent survive the disease but many will be left infertile by radiotherapy and chemotherapy treatment.

The breakthrough has been welcomed by British cancer and fertility doctors. Professor Gedis Grudzinskas, medical director of the Bridge Fertility Centre, London, said: “This is very important because this is the first time that human eggs have undergone changes in the test-tube which normally take place during puberty.

“Prior to puberty the ovaries are not sensitive to hormones. They become sensitive to hormones gradually during puberty. This is a subtle change and it is amazing that this can occur in a Petri dish.”

But Josephine Quintavalle of Comment on Reproductive Ethics, a campaign group worried about some forms of in vitro fertilisation techniques, expressed concern that if the eggs were donated to a woman of childbearing age, a resulting child could have a biological mother who was only a few years older.

Quintavalle said: “Are we going to end up with a child who has a mother who is just six years older? What happens if the child dies? Could the eggs be donated to someone else?

“I don’t think this is the first priority for five year olds. Any intervention for a child going through cancer treatment is an added burden. I feel uncomfortable about this development.”

Doctors have previously frozen the ovarian tissue of several young girls and plan to transplant pieces of the organs back into their bodies when they are older.

So far, however, there have been only two reported pregnancies after ovarian tissue of adults has been frozen and transplanted. The outcome for girls is unknown.

Ovarian tissue could also contain cancerous cells. This risk would be eliminated by freezing and implanting eggs. Across the world more than 100 children have been born from frozen eggs.

Dr Hamish Wallace, a consultant paediatric oncologist at the Royal Hospital for Sick Children in Edinburgh, said: “Maturing in vitro a primitive egg from a five-year-old girl would be a huge advance.

“Doctors wouldn’t need to worry about the ovarian tissue being contaminated with some of the original cancer cells and reintroducing the cancer into the body.”

Israeli scientists have also suggested taking eggs from aborted foetuses. Doctors at the Meir Hospital in Kfar Saba said aborted foetuses could one day become the “mother” of a new baby by donating eggs to an infertile woman.

A woman has her greatest number of eggs, about 5m, as a five-month-old foetus. By the time she is born, about 4m will have disintegrated. By the timea girl reaches puberty, only 300,000 will remain.

From then on, a woman will ovulate one egg each month and have few eggs left by her mid-forties. She will have ovulated about 500 and the remainder will have disintegrated.

The first reported birth from a frozen egg was in 1986. The practice began for medical reasons but is now being used for social purposes.

British women in their thirties who have not yet met their partner or who wish to postpone childbirth to concentrate on their career, are increasingly freezing their eggs. Although scientists initially feared the structure of the eggs would be damaged by the process, many now believe the procedure to be safe and effective.

Eggs are currently frozen using a process known as cryo-preservation, where they are placed in a storage tank containing liquid nitrogen. Scientists have recently developed a more advanced freezing technique, called vitrification, which causes less damage to the eggs.

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