Newcastle University has already produced a human-animal embryo
A Christian organisation will launch legal action in an attempt to overturn a decision to licence scientists to create human-animal hybrid embryos.
Newcastle University and King's College London have had permission to create the embryos for medical research.
The Christian Legal Centre claims the Human Fertilisation and Embryology Authority acted beyond its powers.
The centre insists that existing legislation dating from 1990 does not permit the process.
The group, which includes Christian lawyers and doctors, claims that the HFEA granting the licences was therefore illegal.
It also says that scientific advances have made hybrid embryo use unnecessary.
The centre will ask the High Court to overturn the licences, with which researchers at Newcastle have already produced a human-animal embryo.
Controversial proposals to allow scientists to grow human stem cells inside animal eggs are contained in legislation currently going through Parliament.
The government faced calls from Catholic Church leaders and Catholic Labour MPs for a free vote on the Human Fertilisation and Embryology Bill.
In a compromise, Gordon Brown has said Labour MPs will get a free vote on the most controversial parts of the bill.
However, the prime minister expects all Labour MPs to back the whole bill when it comes to the final Commons vote.
Both the Conservative and Liberal Democrat leaders plan to allow their MPs to have a free vote.
The bill is designed to bring the 1990 regulatory framework for fertility treatment and embryo research in line with scientific advances.
The legal challenge is jointly brought by the campaigning group Comment on Reproductive Ethics (Core).
In a statement, Core said human-animal embryos were neither necessary or desirable, and that the science underpinning them was unlikely to produce useful information or new therapies.
Dr Evan Harris, Liberal Democrat Science spokesman and member of the Commons science and technology select committee, said the legal challenge had no foundation.
He said the committee and the HFEA had received clear legal advice that human-animal embryos were human embryos for the purpose of the 1990 Act, as all their chromosomal DNA is human, and that such research was permitted under the Act and the 2001 therapeutic cloning regulations.