Scientists in Newcastle have scored a world first by creating what they claim is artificial human sperm.
It is currently illegal in the Uk to use artificial sperm to produce a baby
They "prompted" embryonic stem cells to divide their chromosomes, then grow tails and become sperm cells.
It is hoped the technique could eventually allow infertile men to become fathers.
Although it would be illegal in the UK to use the artificial sperm to produce a baby, the scientists behind the technique say they want the law changed to help infertile men.
Professor Karim Nayernia, from the North East England Stem Cell Institute, said his team has no intention of using their breakthrough to produce humans in the laboratory.
But he wants legislation to be put in place "sooner rather than later" to allow the technique to be licenced as a treatment for infertile men.
"This work is a way of investigating why some people are infertile and the reasons behind it," he said.
"If we have a better understanding of what's going on it could lead to new ways of treating infertility."
They have named the cells they created "In Vitro Derived" or IVD sperm.
Other eminent scientists have urged caution saying that the breakthrough has not demonstrated that the artificial sperm cells are capable of producing a viable embryo.
Discovery gives infertile men hope
Dr Allan Pacey, Senior Lecturer in Andrology at the University of Sheffield, said he is unconvinced that the cells produced by Professor Nayernia's group are fully functional sperm cells.
"Whilst 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," he said.
Azim Surani, Professor of Physiology and Reproduction at the University of Cambridge, said the artificial cells are a "long way" from being authentic sperm cells.
Professor Nayernia hit back by claiming his work was being subjected to "unreasonable" criticism.
"Our intention was to open up new avenues of research with these early findings," he said.
"My research group believes this is a very important and exciting step forward and we will continue to develop this area of research."
