The warning came as new research found that live birth rates from fertility treatment were more than 10 per cent higher in the United States than in Britain.

American fertility specialists at the annual conference of the American Society for Reproductive Medicine (ASRM) said women who already had a poor chance of conceiving were being penalised by the policy.

International opinion is divided over the competing ethical issues of giving women the best chance of having a baby, against the risks of multiple births and the rising cost to the state of caring for sometimes severely handicapped babies born as a result.

Since the beginning of last year, in an effort to reduce the rates of twins and triplets born as a result of fertility treatment, the Human Fertilisation and Embryology Authority (HFEA), the Government's fertility watchdog, has allowed a maximum of only two embryos to be implanted at one time into women under 40. Women over 40 are allowed three embryos, but the HFEA is already consulting on whether to limit younger patients to just one per cycle of treatment.

The US has no limits, and on average, three embryos are implanted into a woman each time she has treatment.

The differences in policy and their effects are highlighted by an international study of fertility success rates, to be published soon. Results from the data of the International Committee for Monitoring Assisted Reproductive Technology (Icmart) is to be presented at the conference tomorrow.

They showed that in 2000, just 19.4 per cent of IVF cycles in Britain resulted in a live birth, compared with 31 per cent of American procedures.

Britain had a higher success rate than the European average of 16.4 per cent and the world average of 18.6 per cent, but lagged behind countries such as Slovenia, Denmark and Greece.

Dr David Adamson, chairman of the Icmart committee and vice-president of the ASRM, said the British rules were too strict and were penalising the women who were most in need.

"There is a philosophical difference here between the UK and the US," he said. "People criticise us, but these figures show our success rates are twice as high as some other countries.

"The UK policy may mean fewer multiple births but it also means that women with poor prognosis are receiving a fewer number of embryos that would be appropriate to optimise her chances of pregnancy."

But, he warned that the US was also struggling to reduce its high rate of multiple births, which were linked to increased risks for both mothers and children, as a result of fertility treatment.

In Britain, 26 per cent of successful fertility procedures results in twins, compared with 31.7 per cent in the US.

Dr Adamson added: "There has to be a right balance. I don't think that arbitrary government regulation is the right approach, but then neither is unbridled individual freedom."

John-Paul Maytum, of the HFEA, said: "Our primary role as a regulator is to ensure that IVF treatment is as safe as possible. We know that having a multiple birth is the single biggest risk factor of IVF."

The Icmart study also highlighted the rising popularity of fertility treatment. There are now more than 2,000 clinics in the world, a 20 per cent increase since the committee reported in 1998. Overall, more than 200,000 babies were born as a result of fertility treatment in 2000, a 28 per cent rise in two years.

The conference was also told that women who undergo fertility treatment during the summer are twice as likely to become pregnant as when they try in winter.