New research that could lead to a surge of experiments on monkeys has been strongly condemned by anti-vivisection campaigners.
Scientists in the US announced the birth of the world's first "chimeric" monkeys.
Roku, Hex and Chimero grew from a mixed cocktail of cells taken from a number of different rhesus monkey embryos. Their bodies represent up to six distinct genomes, or genetic codes.
The advance is said to hold "great potential" for future medical research, especially in the field of stem cells.
Chimeric mice have already been used extensively in laboratories, helping scientists to produce genetically engineered animals.
The rhesus monkeys were born after researchers essentially "glued" cells from very early stage monkey embryos together. The "mixed" embryos were then successfully implanted into the wombs of mothers.
"The cells never fuse, but they stay together and work together to form tissues and organs," said lead scientist Dr Shoukhrat Mitalipov, from the Oregon National Primate Centre. "The possibilities for science are enormous."
But the British Union for the Abolition of Vivisection (BUAV) called the research "deeply disturbing".
Dr Jarrod Bailey, the organisation's scientific consultant, said: "Using such highly sentient animals in this research raises enormous ethical concerns and imposes a heavy welfare burden, resulting in severe suffering to many animals.
"As few genetically modified animals show the 'desired' characteristics, many will be killed even before any research can take place, while others will die of severe and unrelated malformations caused by the genetic modifications.
"The monkeys who do exhibit characteristics of 'interest' are destined to suffer greatly by their very nature, and via the experiments to which they will be subjected."
Earlier attempts to create chimeric monkeys by inserting cultured stem cells into embryos failed.
The research, reported in an online edition of the journal Cell, suggests that laboratory-grown primate and human embryonic stem cells (ESCs) may not be as potent as those found inside a living embryo.
ESCs have the potential to turn into any type of tissue in the body. Many scientists believe ESCs, or modified adult cells with similar properties, could revolutionise medical research and regenerative treatments.
But the use of human ESCs in research is controversial because they are obtained from unwanted human embryos.
Dr Mitalipov said: "We need to go back to basics. We need to study not just cultured embryonic stem cells but also stem cells in embryos. It's too soon to close the chapter on these cells.
"We cannot model everything in the mouse. If we want to move stem cell therapies from the lab to clinics and from the mouse to humans, we need to understand what these primate cells can and can't do."
British stem cell expert Professor Robin Lovell-Badge, from the National Institute for Medical Research, said: "Chimeras have been and continue to be a very powerful experimental tool to help understand details of embryo development.
"However, almost all of these details have been determined using the mouse and it has only been an assumption that human and other mammals develop in similar ways. As this paper shows, this assumption is wrong."
Prof Lovell-Badge pointed out there were new methods to make human cell lines which closely resemble mouse embryonic stem cells.
"It is important that the potential of equivalent monkey cells are explored in chimeric embryos," he added.
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