An author of the study says the work was focused on stem cells, and points out that the researchers used an abnormal embryo that could never have developed into a baby anyway.

'None of us wants to make designer babies,' said Dr Zev Rosenwaks, director of the Centre for Reproductive Medicine and Infertility at the New York-Presbyterian Hospital's Weill Cornell Medical Centre.

The study appears to be the first report of genetically modifying a human embryo.

It was presented last autumn at a meeting of the American Society for Reproductive Medicine, but failed to draw widespread public attention at the time.

It did not blow up until the weekend, when Britain's Sunday Times said the British authorities had highlighted the work in a recent report.

Dr Rosenwaks and colleagues did the work with an embryo that had extra chromosomes, making it non-viable.

Following a standard procedure used in animals, they inserted a gene that acts as a marker that can be easily tracked.

He said the goal of the work was to see if the marker would carry into the daughter cells, allowing genetic changes to be traced as cells divided.

He also said that such work could help shed light on why abnormal embryos fail to develop.

But Dr Marcy Darnovsky, associate executive director of the Centre for Genetics and Society, said the Cornell scientists were developing techniques that others might use to make genetically modified people.

She added: 'It's an important ethical boundary that scientists have been observing.

'These scientists, on their own, decided to step over that boundary with no public discussion.'

A London-based group called Human Genetics Alert similarly criticised the work.

Doctors already put foreign genes into people as part of gene therapy to treat diseases. Generally, though, those genetic changes cannot be passed on to future generations because they are made to only certain types of cells, like blood cells or muscle cells.

Genetic changes made to an embryo would theoretically be heritable if the embryo became a baby.

But Dr Kathy Hudson, director of the Genetics and Public Policy Centre in Washington, said she was not troubled by the new direction the researchers had taken.

She said the idea of successfully modifying babies by inserting genes remained a technically daunting challenge, and that it would be ethically unacceptable to attempt to do so when it is unsafe.

'We're not even close to having that technology in hand to be able to do it right,' she said.

Dr Mark Kay, a gene therapy expert at Stanford University, said the Cornell work did not represent a huge technological advance, as the scientists used a common gene therapy technique to ferry the gene into the embryo.

Meanwhile, British legislation allowing human-animal embryo research, which scientists believe could help treat conditions like Parkinson's, but which opponents say is unethical, cleared its first parliamentary hurdle on Monday.

But the Human Fertilisation and Embryology Bill is set to spark heated debate when it returns to the lower house for its second reading.

MPs voted on Monday evening by 340 votes to 262 in favour of sending the Bill to its committee stage where they will argue the ethics involved.

ASSOCIATED PRESS, REUTERS, NEW YORK TIMES