The success of the technique, its simplicity and relatively low cost raise the imminent prospect of a new wave of pilgrims from the ethically disapproving countries of the UK and Europe travelling to the US to secure the baby they want. There is every possibility, however, that the technology will come to the UK.

The procedure takes only a few hours and costs $2,000 (£1,400).

Most of the couples who have visited the Genetics and IVF Institute in Fairfax, Virginia, want a girl or a boy for personal reasons, although the technique is also being used to enable couples to avoid inherited medical disorders that run in one or the other sex in their family.

Harvey Stern, clinical geneticist at the institute, said he had no problem with the ethics of helping couples to have a child of their preferred sex, although he recognised that it would not be acceptable in some other countries.

"I look at this as helping couples to have healthy, wanted children," he said yesterday as he revealed the clinic's latest results at the European Society for Human Reproduction and Embryology conference in Lausanne, Switzerland.

"Many of the couples say, 'I love my sons or daughters, but I would like to have a girl because I want to buy little dresses or I want a boy so I can leave my vineyards to my son.'"

Couples are not accepted for treatment unless they have at least one child of the opposite gender from the one they are seeking.

Selecting the sex of a child by screening embryos created in the laboratory is forbidden in Britain by the human fertilisation and embryology authority (HFEA), unless it is to eliminate the risk of a baby who would inherit a gender-related medical condition, such as haemophilia or muscular dystrophy, which run in boys.

Alan and Louise Masterton, from Monifieth, near Dundee in Scotland, who have four boys and desperately want a girl after their three-year-old daughter Louise was killed in a bonfire accident, are challenging the HFEA under human rights legislation. They say they want Mrs Masterton to be implanted with only female embryos, but the HFEA has made it clear it will not make an exception.

But while the HFEA rules on embryos and sperm donation, there are no rules or regulations to prevent a clinic in Britain offering a sperm-sorting service. There are a couple of clinics which already legitimately do it, but using techniques with much lower success rates. The track record in what are still clinical trials at the Genetics and IVF Institute in Virginia is likely to encourage many more couples to cross the Atlantic. At least one British couple is known to have done so already. The woman, who already had two sons, became pregnant with a girl at her first attempt.

One of the main ethical arguments against sex selection, said Dr Stern, was that it discriminates against women. "But we are doing more girls than boys," he said. The machine, called MicroSort, is more successful at detecting the X chromosome sperm which produce girls. Its success rate for girls is 92%, compared with 72% when it is programmed to select Y chromosome sperm for boys.

About 80% of the couples arriving in Fairfax have wanted to select the sex of their child for "family balancing" reasons, according to Dr Stern, but the other 20% have needed to be sure of the gender because of the danger their baby would inherit a disorder.

Dr Stern told the conference that the MicroSort machine enabled couples to be confident that 90% of the embryos created during laboratory fertility treatment, which mixes the woman's eggs with the man's sperm in a test tube, are female. They are then genetically screened to ensure only female embryos are placed in the womb.

Although the HFEA does not regulate sperm sorting, it is against sex selection on principle, except in the case of inherited genetic disorders. "I think sex selection for family balancing is something the authority is against because it would engender the idea that one sex is more valuable than the other," said its spokesman, James Yeandel.

Ethicists and religious groups are opposed to unnatural selection. Nuala Scarisbrick of Life said it was wrong to reject a potential new human being on the grounds of gender.