Scientists have created the world’s first human-sheep chimera, with the body of a sheep and half-human organs.

The sheep have 15 per cent human cells and 85 per cent animal cells, and their evolution brings the prospect of animal organs being transplanted into humans one step closer.

But the development is likely to revive criticisms about scientists playing God, with the possibility of silent viruses, which are harmless in animals, being introduced into the human race.

Dr Patrick Dixon, an international lecturer on biological trends, warned: “Many silent viruses could create a biological nightmare in humans. Mutant animal viruses are a real threat, as we have seen with HIV.”