Using pigs as a breeding ground for human organs has been branded as "ethically corrupt" and "like something out of fiction" by Dr. Julia Baines from People for the Ethical Treatment of Animals (PETA).
In a bid to overcome a worldwide shortage in human transplant organs, scientists from the University of California have injected human stem cells into pig embryos, to create human-pig embryos known as chimeras.
Researchers are hopeful that the embryos, which are allowed to develop in the sows for 28 days before being removed, will develop as normal pigs - but with one organ being composed of human cells and therefore compatible with the human body.
But Dr. Baines, the science policy advisor for PETA, said the "unethical" practice is cruel to the pigs and needs to be halted.
"Creating human-animal hybrids is a really shocking and crude use of animals which belongs in fictional horror books," she told Julia Hartley-Brewer.
"The mothers undergo invasive procedures to harvest the eggs and create the embryos.
"If those hybrids were allowed to be born, who knows what the unintended painful side-effects would be for the offspring? Tumours, brain defects, deformities, a heightened sense of pain and awareness?
"Animals are not ours to use, for clothing, entertainment, experimentation... We'd be far better off developing and using methods which supersede crude use of pigs and are relevant to human health.
"To travel down this road is ethically corrupt."