The Human Fertilisation and Embryology Bill was never likely to have an easy passage. With its second reading planned for Monday, the Bill is intended to make the existing legislation, which has been in place for 18 years, “fit for purpose in the 21st century”.

Meanwhile, archbishops have spoken up against it, a parliamentary committee has split over it and Ann Widdecombe, the MP, is planning to tour the country to raise public opinion against it.

Because stem-cell research arose from the assisted-reproduction programme, it fits uncomfortably into a law designed to regulate fertility treatment. The Human Fertilisation and Embryology Act 1990 was based on a report by the Warnock committee in 1984. It allowed for the creation of human embryos outside the body and the use of embryos in research, among other things. Additionally, it set up the Human Fertilisation and Embryology Authority (HFEA) to police the law.

“It has been extremely difficult to future-proof this area of the law,” Julian Hitchcock, a solicitor with Mills & Reeve in Cambridge, says.

“The Warnock committee recognised that research was likely to take place, and it was generally a good thing,” says Hitchcock, who is also a director of the East of England Stem Cell Network, which promotes stem-cell research. “But they couldn’t anticipate the sort of thing that would become possible, which is why the HFEA’s remit has had to expand. The law has been struggling to keep up.”

The 1990 Act has been updated piecemeal by amendments and bolstered by new laws, such as the 2001 ban on human reproductive cloning. But rapid scientific progress has become increasingly hard for it to regulate. Under the 1990 Act, for example, an embryo is defined as “a live human embryo where fertilisation is complete”. Since the creation of Dolly the cloned sheep in 1996, science has developed numerous ways of creating embryos without any fertilisation involved.

Controversy has arisen over various sections of the new Bill, including those covering screening and “saviour siblings”, who are created to provide tissue for a sick brother or sister. But the greatest outrage has been caused by the section on stem-cell research.

“The Bill would allow the creation of human admixed embryos – permitting some sort of animal component in what would be generally regarded as a human embryo,” says Professor Martin Bobrow, chair of the Academy of Medical Sciences working group on inter-species embryo research.

The phrase “inter-species embryo” was changed in the Bill to “human admixed embryo,” to emphasise that the created embryos are almost entirely human – although that will not reassure critics who object to any experimentation using human embryos. The type of human admixed embryo likely to be most commonly used – the cyto-plasmic hybrid – will be more than 99 per cent human in genetic terms. It is created by taking the nucleus out of an animal egg and replacing it with the nucleus from a human cell. The embryos will help to allow scientists to understand conditions such as motor neuron disease, and contribute to the development of new drugs.

Researchers say the human admixed embryos are needed to make up for a shortage of human eggs. They have to be destroyed within 14 days, and must not be implanted in a womb.

The mix of human and animal tissue is not unheard of: the 1990 Act lets researchers test whether a man’s sperm is viable by mixing it with a hamster egg to see if the cell divides.

The new Bill has led to the Government coming under fire from both sides of the debate, for either going too far or not going far enough.

The Government originally intended to outlaw the creation of human admixed embryos but under pressure from researchers and a parliamentary committee working on the Bill, most of the main kinds of embryo that scientists currently foresee using have been excluded from the ban.

Under threat of rebellion by some Catholic Labour MPs, Gordon Brown is allowing a free vote on the most controversial sections, including that one.

However, in January, the HFEA granted licences to research groups in Newcastle and London to create human admixed embryos having decided that they counted as human under the terms of the 1990 Act.

The two groups made their applications in November 2006, triggering a lengthy and detailed consideration by the HFEA and legal experts, of whether these fell within the limits of the law.

The new Bill would clarify the situation, making it clear that the HFEA can license the creation of certain types of human admixed embryo.

“Under the 1990 Act, which we have to refer to until any new one comes into place, we have to consider applications in a timely manner,” Paula Woodward, an HFEA press officer, says. “We can’t just leave them dangling until a new law comes in.”

Last month, two ethics groups filed a legal challenge to the Newcastle and London licences, claiming that the HFEA had acted beyond its remit. The HFEA is, however, standing by its decision.

“The Bill tidies up some legalities around the use of early human embryos,” Bobrow says.

“If the law goes through as it stands, and it turns out people don’t need to make human admixed embryos, the HFEA will stop licensing it. The law won’t say you can do it, only that you can ask permission to do it.”