Scientists seeking to create embryos that are part human and part animal will have to wait until next year before beginning work, after a Government watchdog decided to delay a decision on two licence applications.

The Human Fertilisation and Embryology Authority (HFEA) had been expected to approve the proposals this week, but it has postponed a ruling because of the likelihood of a legal challenge.

The Times understands that while the research planned by King's College, London and the University of Newcastle-upon-Tyne has been positively reviewed by independent experts, the HFEA still has questions it wants resolved before permitting the projects. Its licence committee wants to be certain that all aspects of the licences are legally watertight, as embryo rights activists have pledged to take any positive decision to judicial review.

A spokesman for the HFEA confirmed that no decision would be taken this year. “A licence committee has started consideration of the two applications and they are to be considered further in the new year,” he said.

The research, which will make powerful cell models for studying diseases such as Alzheimer's and Parkinson's, has already been agreed in principle by the HFEA. The teams plan to use human DNA with empty cow eggs to make cytoplasmic hybrid or “cybrid” embryos that are genetically 99.9 per cent human.

A public consultation published in September found that there was 61 per cent backing for medical research with cytoplasmic hybrid embryos, leading the HFEA to say there was no fundamental reason why the experiments should not begin once technical issues had been checked.

Opponents of the work, however, are still expected to argue that the HFEA has no powers to approve it, and that it is not necessary or desirable because of the advent of a new technique that could achieve similar aims without animal eggs. Japanese and American researchers last month announced that they have successfully reprogrammed human adult skin cells into an embryonic state, using a method that could potentially make similar tissue models.

This has led Professor Ian Wilmut, of the University of Edinburgh, to abandon plans to use cybrids or cloned human cells for his research, in favour of the new technique.

Other stem cell scientists, however, argue that while this has great promise, it ought to proceed alongside cybrid and embryonic stem cell research and not instead of it.

Cybrids are attractive to scientists because they allow the creation of cloned human embryos without the need for human eggs, which are in short supply. DNA can be taken from patients with conditions such as diabetes or motor neuron disease and used to create cloned embryonic stem cells. These can then be grown into model tissue for studying how these diseases progress, and for developing and testing new drugs.

A year ago, the Government proposed banning such work, but backed down following a revolt by the scientific community.

The King's and Newcastle teams first applied for licences over a year ago, but in February the HFEA decided to conduct a public consultation before ruling. This reported favourably in September.

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