Newcastle University researchers believe the process will help avoid passing on to offspring defective mitochondria genes contained outside the nucleus in a normal female egg.

Mitochondria are a cell's energy source, but mistakes in their genetic code can result in serious diseases such as epilepsy, strokes and mental retardation.

The researchers used a normal embryo created from a man and a woman who had defective mitochondria in her egg.

They then transplanted the embryo into an emptied egg donated by a second woman who had healthy mitochondria.

So far, 10 such embryos have been created.

Although preliminary research has raised concern about the possibility of genetically modified babies, the scientists say the embryos are still primarily the product of one man and one woman.

'We are not trying to alter genes, we're just trying to swop a small proportion of the bad ones for some good ones,' said neurogenetics professor Patrick Chinnery. 'Most of the genes that make you who you are are inside the nucleus. We're not going anywhere near that.'

Only trace amounts of a person's genes come from the mitochondria, and some experts refute the notion spread in media reports that the embryos have three parents.

Prof Chinnery said he hoped the process might eventually be available for parents undergoing in-vitro fertilisation.

Similar research with animals in Japan has already led to the birth of healthy mice which had their mitochondria genes corrected.

REUTERS, ASSOCIATED PRESS