Cambridge University researchers mixed two kinds of mouse stem cell and placed them on a 3D scaffold. After four days of growth in a tank of chemicals designed to mimic conditions in the womb, the cells formed the structure of a living mouse embryo.

The breakthrough has been described as a "masterpiece" in bioengineering that might eventually allow scientists to grow human embryos without sperm or an egg.

Growing embryos would help researchers study the early stages of human life so they could understand why some pregnancies fail but the research is likely to raise questions about what constitutes human life.

Currently scientists can carry out experiments on embryos left over from IVF treatments but they are in short supply and must be destroyed after 14 days.

Scientists say that being able to create unlimited numbers of embryos in the lab could speed up research and perhaps overcome some of the ethical boundaries.

"We think that it will be possible to mimic a lot of the embryological development events occurring before 14 days using human stem cells," said the university's Magdalena Zernicka-Goetz, who led the research.

"We are very optimistic that this will allow us to study key events of this critical stage of human development without having to work on [IVF] embryos. Knowing how development normally occurs will allow us to understand why it so often goes wrong."

The embryos were created using genetically engineered stem cells coupled with extra-embryonic trophoblast stem cells, which form the placenta in a normal pregnancy.

Previous attempts to grow embryos using only one kind of stem cell proved unsuccessful because the cells would not assemble into their correct positions. But scientists discovered that when they added the second "placental" stem cells the two types of cell began to "talk to each other", telling each other where to assemble.

Together they eventually melded to form an embryonic structure, with two distinct clusters of cells at each end and a cavity in the middle in which the embryo would continue to develop. The embryo would not grow into a mouse because it lacked the stem cells that would make a yolk sack.

However, such work raises ethical questions about the "sanctity" of human life and whether it should be manipulated or created in the lab. Critics warn that allowing embryos to be grown for science opens the door to designer babies and genetically modified humans.

David King, director of the watchdog group Human Genetics Alert, said: "What concerns me about the possibility of artificial embryos is that this might become a route to creating genetically modified or even cloned babies.

"Until there is an enforceable global ban on those possibilities, as we saw with mitochondrial transfer, this kind of research risks doing the groundwork for entrepreneurs, who will use the technologies in countries with no regulation."

UK scientists will need to get permission from the Human Fertility and Embryology Authority before attempting to create human embryos using the technique, and experts have called for international dialogue before research can be allowed to progress.

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