The first monkeys cloned in the same way as Dolly the Sheep have been born, raising fears that it may soon be possible to clone humans.
Chinese scientists today announced the births of Zhong Zhong and Hua Hua, a pair of healthy macaques who are completely genetically identical.
The babies are currently being bottle fed and are said to be growing normally compared to monkeys their age. More cloned births are expected in the coming months.
Previously scientists have 'cloned' primates by splitting an embryo in half but the process is essentially just artificial twinning rather than true cloning.
The researchers said they were hoping it would allow the creation of an army of genetically uniform monkeys which could be used in labs to improve research.
The study was greeted with mixed opinion from British scientists and ethics charities.
Prof Darren Griffin, Professor of Genetics at the University of Kent, said: "The first report of cloning of a non-human primate will undoubtedly raise a series of ethical concerns, with critics evoking the slippery slope argument of this being one step closer to human cloning.
"The benefits of this approach, however, are clear. A primate model that can be generated with a known and uniform genetic background would undoubtedly be very useful in the study."
However, the Scottish Council on Human Bioethics warned the research 'opens the door' to human cloning.
Dr Callum MacKellar, Director of Research of the SCHB, indicated that: "There is indeed a very serious risk that such human clones would just be created to fulfil the desires of their creators.
"Human beings should never be brought into existence to just replace a person who already exists or who has died because some parents want a copy of a famous or desirable person.
"Children should be created for themselves no matter who they are."
Human cloning is currently banned under international law and Prof Robin Lovell-Badge, Group Leader of the Francis Crick Institute in London, warned it would be 'foolish' to attempt to clone humans using the same technique.
"It would be far too inefficient, far too unsafe, and it is also pointless," he said. "Clones may be genetically identical, but we are far from only being a product of our genes."
For the new experiments scientists took donor eggs from monkeys and removed the nucleus before adding tissue cells from a macaque foetus, and implanting them into surrogate monkeys.
Although Zhong Zhong and Hua Hua are genetically identical, having received DNA from the same monkey, they were born two weeks apart to different surrogate mothers.
The same technique was used by the Roslin Institute in Edinburgh to create Dolly the Sheep in 1996, the first mammal to be cloned. Dolly was euthanised on February 14, 2003, because she had a progressive lung disease and severe arthritis.
Until now attempts to carry out the same process in primates have proved tricky because of the far more complicated process of cell division and early development.
The Chinese team solved the problem by switching on and off certain genes which were preventing the embryos from forming properly.
Although the technique worked when using cells from a monkey foetus, scientists found that when they attempted the same process using adult cells, the baby monkeys only lived for a few hours after birth. And out of 79 attempts, only two babies were born.
"We tried several different methods, but only one worked," said Qiang Sun, Director of the Nonhuman Primate Research Facility at the Chinese Academy of Sciences Institute of Neuroscience. "There was much failure before we found a way to successfully clone a monkey."
However, critics questioned whether the experiments were ethical.
"Cloning these monkeys involved many failed attempts, which causes enormous suffering to the animals involved," said Dr Helen Wallace, Director of GeneWatch UK.
"Given the serious problems with this technique, we question whether it is ethical to be creating monkey clones."
Dr Julia Baines, Science Policy Adviser at PETA UK added: "Cloning is a horror show: a waste of lives, time, and money – and the suffering that such experiments cause is unimaginable.
"Because cloning has a failure rate of at least 90 per cent, these two monkeys represent misery and death on an enormous scale. This experiment – and all other experiments on animals – should be ended immediately."
The researchers say they plan to continue improving the technique so that more monkeys survive and are monitoring Zhong Zhong and Hua Hua's physical and intellectual development.
The research was published in the journal Cell.
Are we close to cloning humans
Sarah Knapton, the Daily Telegraph's Science Editor on cloning
It is more than 20 years since The Roslin Institute in Edinburgh succeeded in cloning Dolly the Sheep. Yet, until today, nobody has succeeded in cloning a primate.
Primates are the same order as humans, so the announcement inevitably raises the question of whether human clones will be next.
Although researchers at the Chinese Academy of Sciences insist the current goal is simply to create genetically identical monkeys to help make laboratory research more consistent, it is inconceivable that China is not, at least, considering cloning humans.
In recent years China has been forging ahead with controversial experiments which are banned elsewhere, leading to accusations that the country is becoming the 'Wild West' of science.
Only last week, Chinese doctors told the Wall Street Journal that they had already genetically engineered 86 cancer and HIV patients, 15 of whom have since died. None of this work has been published or peer reviewed, so the scientific world is largely in the dark about these developments.
When it comes to human cloning it was previously thought that a combination of scientific, ethical and commercial hurdles would prevent the controversial practice.
Yet today one of those barriers has been removed. If it is possible to recreate the cell differentiation which allows the formation of a primate embryo, then the ability to do the same with a human nucleus cannot be far behind.
The sheer cost of cloning, which has made it an unviable prospect for most organizations, is is also unlikely to prove a problem for the Chinese. It is illuminating that the newborns have been named Zhong Zhong and Hua Hua - named for the adjective 'Zhonghua' which means Chinese nation.
Late last year, scientists at the Harbin Medical University, in China carried out the first head transplant on a dead body and say they will soon be ready to attempt the procedure on a live patient.
Italian Professor Sergio Canavero, Director of the Turin Advanced Neuromodulation Group, who has been working with the Harbin team, told The Telegraph that he was hoping successful head transplants would eventually lead to humans being able to clone their bodies and replace them once they became too old, or diseased.
Although that kind of Frankenstein science is likely to be decades away, the prospect of cloned humans has today undoubtedly moved a step closer.