A team of British scientists in Newcastle said they developed a new technique that allows the creation of human sperm in the laboratory using embryonic stem cells.

Professor Karim Nayernia at Newcastle University and the NorthEast England Stem Cell Institute (NESCI) said, "This is an important development as it will allow researchers to study in detail how sperm forms and lead to a better understanding of infertility in men -- why it happens and what is causing it."

"This understanding could help us develop new ways to help couples suffering infertility so they can have a child which is genetically their own."

Prohibited by law, they assured that the sperm, developed from stem cells with XY chromosomes (male), would not be used for fertility treatment.

Detailing the procedure the researchers said they began with stem cell lines derived from human embryos donated following IVF treatment which had been removed when the embryo was a few days old and were stored in tanks of liquid nitrogen.

The stem cells were brought to body temperature and put in a chemical mixture to help them grow and were "tagged" with a genetic marker which enabled the scientists to identify and separate so-called "germline" stem cells from which eggs and sperm are developed.

The male, XY stem cells underwent "meiosis", or cell division which then produced "fully mature, functional sperm." The entire process of creating and developing the sperm took four to six weeks.

The Newcastle team in the journal Stem Cells and Development, said it would be at least five years before the technique is perfected.

Professor Nayernia said, "It will also allow scientists to study how cells involved in reproduction are affected by toxins, for example, why young boys with leukaemia who undergo chemotherapy can become infertile for life - and possibly lead us to a solution."

On the issue of ethics Professor Nayernia stressed the researchers had no intention of "producing human life in a dish".

Dr Allen Pacey, senior lecturer in andrology at the University of Sheffield said, "As a sperm biologist of 20 years' experience, I am unconvinced from the data presented in this paper that the cells produced by Professor Nayernia's group from embryonic stem cells can be accurately called 'spermatozoa'."

"While the cells produced may possess some of the distinctive genetic features and molecular markers seen in sperm, fully differentiated human spermatozoa have specific cellular morphology, behaviour and function that are not described here."