For the first time ever researchers have kept an embryo in a lab alive beyond the point it would normally implant in the womb.

The research was halted shortly before the embryo reached 14 days old, the legal limit in many countries including the UK and US. The experiment went far beyond anything done so far.

The result was hailed as a milestone by some scientists who have said the research could revolutionise our knowledge about the earliest stages of human life and improve fertility treatments.

However the breakthrough has raised ethical concerns among some scientists. It has also led to immediate pressure for the 14 day limit to be extended because the law restricted what scientists were able to achieve.

A number of Christian scientists and bio-ethics experts have expressed concerns.

John Bryant, emeritus professor of cell and molecular biology at Exeter University, told Christian Today he would be anxious about extending the deadline.

A practising Christian, Bryant says he thinks an embryo only has potential to be a person once it has implanted in the womb. He acknowledged he was coming from a more liberal Christian position and praised the research as "remarkable" and "very clever". But Bryant called for "ethical caution" around extending the 14 day limit because it goes well beyond when an embryo would normally implant.

He pointed to ectogenesis research which tries to develop a pregnancy outside the natural womb. "Such research is totally wrong," said Bryant. "It may well have good outcomes for a limited number of people but just because we can does not mean we should."

Philippa Taylor, head of public policy for Christian Medical Fellowship, holds to a more conservative viewpoint on embryonic research. She told Christian Today there were "two big concerns" with the study.

"Firstly we need to have a debate about the ethics of researching on embyros and whether they should have any special status," she said. 

"Secondly we need to discuss the public involvement on issues like this and who gets to make the decisions."

She said the current 14 day limit was "arbitrary" but added "most would agree we need boundaries. The challenge is where to draw the line."

"I would be very concerned about extending the limit as these [embryos] are very young human beings. The 14 day limit itself acknowledges the special status of embryos which must be maintained."

Taylor is also medical ethics adviser to the public policy charity CARE. Chief executive Nola Leach told Christian Today the debate was about "competing ideologies".

She said: "The special status once enjoyed by human embryos in the eyes of policy makers has tragically disappeared and now, human embryos are viewed just as a collection of cells."

Leach added: "Whether we push past the 14 day limit or not, we would still have fundamental ethical concerns about the way human embryos are being treated because a human embryo is a human life and therefore is intrinsically valuable."

Rev Dr Brendan McCarthy, the Church of England's national adviser on medical ethics, welcomed the opportunity for further debate. However, he said: "The present limit recognises the importance of the emergence of the 'primitive streak' heralding the beginning of neural development.

"If the beginnings of neural development do not provide a watershed for research, it is difficult to see where else the limit might be placed in order properly to respect the developing human embryo and foetus."

Professor David Jones, director of the Roman Catholic Anscombe Bioethics Centre, agreed with Taylor that the 14 day limit was "completely arbitrary". Jones, who holds that all research on embryos is unethical, said: 

"In the future this may even be seen as the first step towards culturing babies outside the womb, where the child is not only conceived outside the protection of his or her mother's body but no such protection is even envisaged at any stage.

"Human life and human pregnancy should not be separated in this way. On a technical level it is a scientific breakthrough but it is also a further step away from humane and ethical science and a further step towards an increasingly inhuman future."

A number of scientists, including the leading geneticist Professor Robin Lovell-Badge, have called for a public debate on extending the limit. Additionally The Nuffield Council on Bioethics will hold a meeting of experts "to evaluate whether, after 25 years, there may be persuasive reasons to review this legal limit or whether the reasons for its introduction remain sound".

Whether or not scientists see the need for a change in the limit, the decision is enshrined in law and any change will have to pass through parliament. This will allow for public involvement in the debate before any extension is considered. It is worth bearing in mind, however, that the limit only applies in a handful of countries. There is nothing to stop scientists carrying out the research where regulations are less strict.