British scientists say they have successfully cloned a human embryo - the country's first.
The Newcastle University team took eggs from 11 women, removed the genetic material and replaced it with DNA from embryonic stem cells.
Three of the resultant clones lived and grew in the laboratory for three days and one survived for five days.
The critical factor for success appeared to be how quickly the egg was collected and manipulated.
We are talking about several years before we are talking about a cell-based therapy that can go back into the patient
Professor Alison Murdoch
Patient-specific stem cell first
Any longer than an hour and there was no success, Professor Alison Murdoch and colleagues found.
The clone that lasted for five days had been collected and manipulated within 15 minutes.
The ultimate aim of the research is to make cloned embryos from which stem cells can be harvested and used to treat diseases.
Stem cells have the ability to develop into virtually any tissue in the body and could, in theory, be used to replace damaged cells in conditions such as Parkinson's disease and diabetes.
But Professor Murdock said this was still a long way off.
"We are talking about several years before we are talking about a cell-based therapy that can go back into the patient.
Colleague Dr Miodrag Stojkovic said: "I'm really happy but I know that this is just the beginning of a long journey so we have to continue to try to derive stem cells that will definitely help us one day to cure diseases."
Therapeutic cloning is allowed in Britain, although reproductive cloning - the cloning of human embryos with the intention of creating a baby - was made illegal in 2001.
The UN recently voted in favour of a ban on all human cloning, but this was non-binding which means the UK can continue to do therapeutic cloning.
But cloning opponents criticised the research.
Julia Millington from the ProLife Alliance said cloning for research purposes was profoundly unethical.
Josephine Quintavalle from CORE said: "No matter how it is created, a human embryo's destiny should be to live and not to be turned into human stem cells."
Life said cloning was "unsafe and inefficient", and involved exposing women to dangerous fertility drugs in order to collect sufficient eggs.
South Korean scientists cloned 30 human embryos last year.
However, they took the genetic material from normal cells from the same women who were the egg donors and combined it with these eggs.
The UK research is published in Reproductive and BioMedicine Online.
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