A woman of 58 is to become one of Britain's oldest mothers after having fertility treatment at a controversial private clinic, it emerged yesterday.

Critics warned that treating women past the natural age of menopause could pose ethical issues as well as leading to medical problems.

But Ian Craft, director of the London Fertility Centre, said he was "delighted" that his patient, known as Sandra, was to give birth shortly. Her age indicates that she became pregnant via egg donation. Her case satisfied the criteria laid down by the human fertilisation and embryology authority.

Sandra is the third woman over 55 to hit the headlines following treatment at the clinic, thought to be the only unit in the UK helping women of that age.

In 1997, a Welsh hill farmer Liz Buttle, 60, become the oldest mother in Britain after lying about her age to receive treatment. Two years ago Lynne Bezant, 56, and her husband Derek, 55, had twins following treatment at the clinic. Other units have lower age limits, usually about 50, and most health authorities do not fund treatment for women over 36.

"I think we should be encouraging responsible practice in the UK, which would be to confine treatment to women below the age of menopause," said Richard Kennedy, secretary of the British Fertility Society.

"Most fertility specialists regard 50 as the upper level for treatment. The physiological changes of pregnancy are tremendous. The workload on the heart increases by 40-50% during pregnancy.

"Presumably this woman is pretty fit for her years. But she is more likely to have high blood pressure and complications which would place a tremendous physical strain on her. There are also concerns with the welfare of the child."

The HFEA, which regulates fertility treatment, does not specify an upper age limit for assisted conception. But it states that a couple's health, ages and "likely future ability to look after or provide for a child's needs" should be considered by the ethics committees which decide whether clinics can treat patients.

The world's oldest mother is thought to be Rosanna Dalla Corta, from Italy, who gave birth in 1994 at the age of 63.