Scientists have been accused of paving the way to living human clones after creating two genetically identical monkeys.
Long-tailed macaques Zhong Zhong and Hua Hua were born eight and six weeks ago respectively at a laboratory in China.
They are the first primates to be cloned using the DNA-transfer technique that produced Dolly the Sheep 20 years ago.
The aim of the scientists was to open the door to populations of genetically uniform monkeys that can be customised for ground-breaking research into human diseases.
But campaigners have hit out at the research, claiming it was a step towards the eventual cloning of humans and involved unacceptable cruelty to the animals.
Dr David King, director of the lobby group Human Genetics Alert, said: "We are concerned that this is a stepping stone to the creation of human clones
"Although it looks like that would be technically difficult, those with enough financial resources and the ambition to be the first to create a cloned child are likely to try.
"There would undoubtedly be a market for human clones."
He called for an international ban on the cloning and genetic engineering of humans.
Dolly made history in 1997 after being cloned at the Roslin Institute in Edinburgh.
It was the first time scientists had managed to clone a mammal from an adult cell, taken from the udder of a Finn Dorset sheep.
Since then many other mammals have been cloned using the same somatic cell nuclear transfer (SCNT) technique, which involves transferring cell nucleus DNA to a donated egg cell that is then prompted to develop into an embryo.
They include sheep, cattle, pigs, dogs, cats, mice and rats - but until now, there has never been an SCNT-cloned monkey.
The Chinese team led by Dr Qiang Sun, director of the Non-Human Primate Research Facility at the Chinese Academy of Sciences Institute of Neuroscience, Shanghai, made the breakthrough by using DNA from foetal connective tissue cells.
After the DNA was transferred to donated eggs, genetic reprogramming was used to switch on or off genes that would otherwise have suppressed embryo development.
Hua Hua was born six weeks ago (Qiang Sun and Mu-ming Poo/Chinese Academy of Sciences/PA) [/caption]
Zhong Zhong and Hua Hua were the result of 79 nuclear transfer attempts. Two othermonkeys were initially cloned from a different type of adult cell, but failed to survive.
Dr Katy Taylor, from the anti-vivisection group Cruelty Free International, said: "The scientists themselves admit that their work has involved the abnormal development and death of many monkeys before and after birth.
"The scale of suffering and death of these highly intelligent and sensitive animals is substantial."
The research is reported in the latest edition of the journal Cell.
Cloned monkeys would allow scientists to study "a lot of questions" about primate biology, Dr Sun stressed.
He added: "You can produce cloned monkeys with the same genetic background except the gene you manipulated. This will generate real models not just for genetically based brain diseases, but also cancer, immune or metabolic disorders and allow us to test the efficacy of the drugs for these conditions before clinical use."
The scientists insisted they followed strict international guidelines for animal research, set by the US National Institutes of Health.
British cloning expert Professor Robin Lovell-Badge, from The Francis Crick Institute, London, said he did not believe the research increased the chances of humans being cloned.
He said: "The work in this paper is not a stepping-stone to establishing methods for obtaining live-born human clones.
"This clearly remains a very foolish thing to attempt. It would be far too inefficient, far too unsafe, and it is also pointless."
Technically Zhong Zhong and Hua Hua are not the first primate clones. That title goes to Tetra, a rhesus monkey born in 1999 through the much simpler method of embryo splitting that does not employ DNA transfer.
It is the same process that leads to the birth of natural twins, but can only generate up to four offspring at a time.