Women can be five times more likely to become pregnant depending on which fertility clinic they choose, figures show. More than half had a baby after treatment at some clinics compared with 10 per cent at others.

But the regulatory authority warned that some clinics may be boosting rates by routinely transferring two or more embryos during IVF treatment, increasing multiple births.

Dame Suzi Leather, the chair of the Human Fertilisation and Embryology Authority (HFEA), warned that multiple births represent the biggest health risk for women and their unborn children when having fertility treatment.

She said patients needed to be made more aware of the dangers, and more pressure must be put on doctors to reduce the number of embryos they transfer. But she indicated that the lack of NHS funding for treatment meant it was unlikely a mandatory rule of single embryo transfers (SET) would be imposed soon.

Data on all clinics licensed by the HFEA is available on the authority website: www.hfea.gov.uk

More than 10,000 babies were born in the UK as a result of fertility treatment between 2003 and 2004. The overall live-birth rate rose on the previous year, from 20.4 to 21.6 per cent per treatment cycle.

For women under 35 using their own fresh eggs, the Assisted Reproduction and Gynaecology Centre (ARGC) in London has the highest success rate, with 53.8 per cent having a baby after a treatment cycle. The Cromwell IVF and Family Centre in Darlington has the lowest rate at 10.7 per cent.

But the ARGC also has the highest rate of multiple births, with 40.5 per cent of all children born to women under 35 being a twin or triplet. No twins or triplets were born at the Cromwell's Darlington clinic.

Nearly one in four of all IVF births result in twins or triplets. Women carrying twins or triplets have a much higher risk of complications.

The risk of a baby dying before birth or during the first week of life is four times higher for twins and seven times higher for triplets. The incidence of cerebral palsy is five times higher among twins and 18 times higher among triplets.

Under HFEA guidelines, clinics can transfer two embryos at most into women under 40 or three for older patients. Only 2 per cent of the Cromwell Clinic's cycles involve three embryos, compared with 39 per cent for the ARGC.

Dame Suzi said: "I don't think it is difficult to understand that if you are desperate for a baby and you have tried for years ... it may seem a wonderful option for two or three to arrive at the same time. But patients need to question whether it's really sensible to want a baby at any cost."

Mohammed Taranissi, director of the ARGC, said: "The simple answer to why our success rate is so high is that we treat patients as individuals. We don't have standard protocols for people; we make individual assessments. I am very keen on single-embryo transfers but that is because we are very good at implanting embryos ."

Some European countries have a mandatory policy of transferring only one embryo per cycle. But Dame Suzi said she favour a more collaborative approach. She admitted a problem with a policy of single transfers was lack of funding.

A spokesman for the Cromwell clinic said yesterday: "The period these figures refer to are from when the clinic was first launched." He added: "We now have a success rate of about 38 per cent in the younger age group."