A controversial scientist, who failed in his attempts to clone a human in January, has met further scepticism over his latest cloning claims.
Dr Zavos' cloning work has evoked wide criticism
US fertility doctor Panos Zavos says he has created a cloned embryo using tissue from dead people.
Experts said such actions would exploit the vulnerability of grieving people who had been bereaved.
And the Royal Society also questioned "a lack of evidence" behind Dr Zavos' claims.
Dr Zavos told a press conference in London he had successfully combined genetic material from three dead people with cow eggs to make embryos that were an identical copy of the deceased.
He said he took DNA from blood samples from an 11-year-old girl who had died three days earlier in a car crash.
The other corpses whose tissue he took included an18-month-old toddler who had died following surgery, and a 33-year-old man.
Two of the three experiments were successful, creating embryos that Dr Zavos claimed would be "potentially viable" if left to grow in the human womb.
Dr Zavos said he had not done this yet and had stopped the embryos' development at an early stage when cells begin to divide and multiply rapidly.
But he said his current work was a major step forward, showing this could be a way to clone humans in the future.
He said similar studies had been carried out successfully in animals using dead tissue with good results.
He said the animal offspring produced had none of the deformities reported with other cloning methods.
Other scientists condemned Dr Zavos' actions.
Professor Richard Gardner, chair of the UK's Royal Society working group on stem cell research and cloning, said: "The work using human genetic material and cow eggs that Dr Zavos claims to have carried out would not be allowed under British law and is both scientifically questionable and ethically unacceptable.
"It is grossly misleading to suggest that you can replicate a loved one, such as a child lost in a road accident, by producing a cloned person with the same genetic material."
He said it was impossible to evaluate Dr Zavos' claims because his work had not been checked through proper scientific peer review.
Professor Gardner said: "Even more worrying is that Dr Zavos sees this work as a first step towards human reproductive cloning, which he still appears to want to carry out.
"The scientific community, and society as a whole, should be concerned about this because current evidence shows reproductive cloning is medically unsafe, scientifically unsound and socially unacceptable."
Dr Zavos denied suggestions that his work was unethical and exploited grieving parents.
He said all of the relatives involved knew they were taking part in research and there was no prospect at this stage of them getting new cloned babies.
"I'm not in the business of exploiting anyone. I have never done that. There are sensitivities here that we've dealt with in a professional manner."
He said the aim in the future would be "to replace the child and not to resurrect the child".
Dr Simon Fishel, managing director of the Centres for Assisted Reproduction (CARE) group of fertility clinics, said it was time for a worldwide ban on reproductive
"This would remove the false hope given by mavericks to patients."
He said using human DNA in a cow's egg would only create confusion rather than understanding of reproductive technology.
"At worst this is misleading and exploitative to the patients funding the research," he said.