The procedure appears to restore the softness and suppleness of breast tissues, undoing the damage frequently caused by lumpectomy and radiotherapy. Early signs indicate that it also eases the considerable pain with which patients are often left after treatment.

More than 31,000 women a year in Britain with early-stage breast cancer undergo operations in which just the lump and a healthy margin of tissue around it are removed. The cavity left in the breast following surgery can vary from a dimple to a mini-crater, but the dip invariably becomes more pronounced following radiotherapy, which most patients need. Irradiation damages the blood supply to the breast and shrinks and toughens overlying skin so that it sticks to the chest wall. Nerves can get trapped in the resulting scar tissue, causing constant discomfort. Although some surgeons have had short-term success with simple fat transfers - liposuctioning fat from elsewhere and injecting it into the breast hollow - the blob of fat struggles to get a decent blood supply.

The alternative has been more surgery, either to reduce the size of the healthy breast so it is a better match with the other or, perhaps most upsettingly, a total mastectomy and full breast reconstruction on the lumpectomy side. Scientists believe the stem-cell fat will reduce the need for this sort of revision surgery by suppressing inflammation and encouraging the laying down of a healthy blood supply so that fat loss is minimised.

The organs and tissues of the body are a natural cache of adult stem cells ready to repair damage when injury occurs. Ironically, it is the chameleon-like attributes of stem cells that fuelled doubts in the minds of some British surgeons before the start of the trial last summer. "Cancer is all about the proliferation of cells, so our theoretical worry was that we could be putting cells into the breast that would encourage rapid growth of surrounding tissue, resulting in another cancer," said Philip Turton, a consultant breast surgeon at Leeds General Infirmary.

Lead investigator and consultant plastic surgeon Eva Weiler-Mithoff says she is impressed with the results so far. "What is striking is the softness and suppleness the technique gives the skin and tissues. When I see these stem-cell-enhanced patients after three months, their skin is significantly softer."

Patients have their fat harvested under general anaesthetic. Half is set aside; half is washed in a processor, where the stem cells are distilled. This concentrate is then mixed with the set-aside fat and injected under local anaesthetic.

Surgeons had two more nagging concerns before the study got under way - that cysts could develop in the breast if the new fat liquefied and that dying fat could misleadingly show on a mammogram as tiny specs of calcification, the first sign of breast cancer, giving rise to false positives.

But if, as expected, results show that more than 90% of the fat survives, then any doubts should be quashed. Weiler-Mithoff said: "This technique should help us to restore a natural-looking breast with minimal risks. Patients don't need a big operation and they'll be left with no scars. It's a win-win."