Britain's first human-animal hybrid embryos have been created, forming a crucial first step, scientists believe, towards a supply of stem cells that could be used to investigate debilitating and so far untreatable conditions such as Alzheimer's disease, Parkinson's and motor neurone disease.

Lyle Armstrong, who led the work, gained permission in January from the Human Fertilisation and Embryology Authority (HFEA) to create the embryos, known as "cytoplasmic hybrids".

His team at Newcastle University produced the embryos by inserting human DNA from a skin cell into a hollowed-out cow egg. An electric shock then induced the hybrid embryo to grow. The embryo, 99.9% human and 0.1% other animal, grew for three days, until it had 32 cells.

Eventually, scientists hope to grow such embryos for six days, and then extract stem cells from them. The researchers insisted the embryos would never be implanted into a woman and that the only reason they used cow eggs was due to the scarcity of human eggs.

The team's success comes days after Gordon Brown was forced to give MPs a free vote on the human fertilisation and embryology bill, which has faced condemnation from Catholic bishops. Cardinal Keith O'Brien used his Easter sermon to denounce what he called experiments of "Frankenstein proportion" and called the bill a "monstrous attack on human rights, human dignity and human life".

Catholics object to the idea of putting human and animal DNA in the same entity and to the notion of creating what they regard as a life for the purposes of research, a life that will then be destroyed.

John Burn, head of the Institute of Human Genetics at Newcastle University, said the embryos had been created purely for research. He told the BBC's Six O'Clock News last night: "If you look down the microscope it looks like semolina and it stays like that. It's never going to be anything other than a pile of cells. What it does is give us the tools to find out the simple questions: how can we better understand the disease processes by working with those cells in the body?"

The research has not yet been published, but the team plans to submit the work for peer review in the coming months. Other scientists welcomed the work but also urged caution in interpreting the results.

Colin Blakemore, a former head of the Medical Research Council, said: "The creation of hybrid embryos is not illegal and researchers in Newcastle and London were granted provisional licences for such research in January, after extensive consultation by the HFEA ... This research is at a very early stage and no results have been peer-reviewed or published.

"However, these preliminary reports give hope that this approach is likely to provide stem cells for research without the use of human eggs or normal human embryos. The new bill is intended to confirm the arrangements for regulation of this important area of research."

Mark Walport, director of the Wellcome Trust, agreed that it was too early to assess the significance of the results. "The aim of the research is to advance human health," he said. "This work emphasises the importance of the parliamentary scrutiny of this area of research over the coming weeks."

Peter Andrews, professor of biomedical science at the University of Sheffield's Centre for Stem Cell Biology, said: "The production of embryos by transferring the nucleus of an adult human cell to a human egg ... has already proved very difficult, let alone combining a human nucleus with an animal egg. Apparently these researchers have achieved some success - but by using the nucleus from a very early embryonic cell, which might be easier to reprogramme than an adult cell. At the moment it is impossible to assess the significance of this report until we know more details of what has been achieved ... the results have been repeated and, importantly, they have been reviewed by independent researchers in the usual way."

Josephine Quintavalle, of the pressure group Comment on Reproductive Ethics, said the research should not worry those opposed to hybrid embryos because the Newcastle work did not seem convincing. "The embryos didn't survive, they were created from embryonic stem cells rather than adult tissue, and there's a lot of question marks over the research."

But she added: "What it has done is wake up the public to this reality, that while parliament is getting in a tizz about this, while the whole country is up in arms discussing it, the HFEA is already issuing licences."

Evan Harris, the Liberal Democrat MP who led a campaign to ensure that research on human-animal hybrids was not banned by parliament said: "Creating these sort of cytoplasmic hybrid embryos was deemed legal and legitimate under the 1990 HFEA Act and the 2001 Therapeutic Cloning Regulations by both the HFEA and by the science select committee, and was approved on that basis by the HFEA after a public consultation and after approval by a further unanimous select committee report. Therefore it is wrong to say that this is pre-empting parliamentary debate or votes on the new legislation in this area - the statutory framework is being updated."

FAQ The controversy

What is the purpose of the research?

Researchers say the creation of hybrids is essential to advance research into conditions such as Alzheimer's, Parkinson's and motor neurone disease.

Why has it caused such a row?

Religious groups object to the notion of putting human and animal DNA in the same entity. Cardinal Keith O'Brien denounced what he called experiments of "Frankenstein proportion".

Why has it been allowed to go ahead?

The Human Fertilisation and Embryology Authority ruled that the research should be allowed, following a three-month public consultation.

Martin Hodgson