Scientists have created synthetic human embryos – using no eggs or sperm – provoking deep ethical questions, according to reports.The synthetic embryos – only days or weeks old – could help researchers study the earliest stages of human development and explain pregnancy loss.

Nobody is currently suggesting growing them into a baby.But the rapid progress has outpaced discussions on how they should be dealt with ethically and legally.

Prof James Briscoe, from the Francis Crick Institute, said the field needed to “proceed cautiously, carefully and transparently” to avoid a “chilling effect” on the public.

The development of human synthetic embryos was announced at the annual meeting of the International Society for Stem Cell Research.Synthetic embryos are also known as “embryo models”, as they resemble embryos, for the purposes of research, rather than being identical to them.The work comes from the laboratories of Prof Magdalena Zernicka-Goetz, from the University of Cambridge and the California Institute of Technology.

The full details have yet to be published and made available for scientific scrutiny, leading many researchers to feel unable to comment on the significance of the reports.

But the principle is the synthetic embryos are made from a stem cell rather than a fusion of egg and sperm.