The past few months have seen Britain's politicians tying themselves in knots over the question of whether to allow the creation of 'hybrid' embryos, those made from both human and animal material, for research purposes. Meanwhile, the embryologists who have applied for permission to carry out this research have waited patiently (or perhaps not so patiently) for the verdict.

The outlook initially looked bleak. In December, the government published a policy outline proposing a ban on virtually all forms of hybrid embryos. Medical research organizations reacted angrily, and the House of Commons Select Committee on Science and Technology began working on a report, unveiled in April, criticizing the proposed ban as unnecessary and unfairly restrictive.

Last week, the verdict arrived in the form of the government's draft bill, which seems to be a turnaround on the issue. If it becomes law, the new legislation looks set to allow the creation of 'cybrid embryos' — a particular form of hybrid in which human DNA is placed in an empty animal egg — by the two British research groups that have applied to do it. Assuming that the groups ultimately receive licences to create these embryos, they should give rise to valuable stem cells that could be used to study conditions such as Parkinson's disease.

A range of other techniques also look set to be approved, including the creation of human embryos with animal genes inserted in their DNA, human embryos containing animal cells, and genetically engineered animals with human genes (although the latter will now fall under the purview of the government's animal-research guidelines). Researchers applying to create these specific entities look set to have their requests granted, within the existing rules that no in vitro human embryo should be allowed to develop beyond 14 days, and no embryo derived from animal material should be implanted in a human uterus.

In the main, research advocates are satisfied with the proposal, and the government is to be applauded for not persisting with its plan for an outright ban. But a closer reading of the draft bill reveals that the proposed legislation is prescriptive, in mind-boggling detail, rather than truly permissive — and this is an approach that looks set to harm the field of embryology in the longer term.

The bill states that the creation of hybrid embryos “should not be permitted but that there should be a regulation-making power allowing exceptions to the prohibition”. That power will be the Human Fertilisation and Embryology Authority (HFEA, which is to be rebranded as the Regulatory Authority for Tissue and Embryos), and those exceptions will be the various strictly defined techniques prescribed in the draft act.

But politicians are not embryology experts, and in attempting to compose a definitive list of acceptable techniques, they risk saddling researchers with a piece of legislation that does not allow the freedom to pursue new and promising possibilities not covered by the draft bill. And no matter how thoroughly you consult stakeholders now, someone will come along in five or ten years' time and ask for permission to do something you hadn't thought of.

It was just such a situation that caused the recent ill-feeling over the government's handling of hybrid-embryo proposals. When separate research groups at Newcastle University and King's College London asked the HFEA last year for permission to create cybrid embryos, the agency panicked and referred the issue to the government, which proposed its infamous ban before later admitting that such research is necessary and useful. The proposed legislation will prompt a repeat performance every time researchers propose something that the regulatory body does not feel comfortable dealing with — and by inviting politicians into the fray with greater regularity, it encourages repeated attacks by those who want to see all work on human embryos outlawed.

Much better would be to preserve the spirit of Britain's 1990 Human Fertilisation and Embryology Act, which the draft bill is intended to replace. That act contained several general rules of thumb that have provided a useful ethical framework while not stifling research. Even 17 years down the line, those rules still stand up to scrutiny. Embryologists could look forward to a more fruitful future if they were given a regulatory body with the ethical muscle to approve novel techniques while adhering to tried and trusted principles.