Tens of thousands of human tissue samples could be used in experiments involving animal-human embryos without the consent of the patients who donated them.
Under new rules coming into force next month, scientists will have access to huge stores of samples collected by hospitals and tissue banks.
They must try to get the consent of people who donated samples before 1 October, but the experiments can still go ahead even if they cannot be tracked down.
Joyce Robins of Patient Concern, a pressure group, said many people had “not an inkling” about the changes.
“This is absolutely frightening”, she told medical journal The Lancet.
“People who have donated for medical research may well not agree with human-animal hybrids, which are one of the most controversial ideas out there.
“Scientists know how hard it would be to get consent for these kinds of experiments – this is an attempt to get around the obstacles.”
One major research project called UK Biobank already has a collection of tissue samples from 363,000 people.
It has refused to provide an undertaking that it will not provide samples for animal-human embryo research without the consent of the donor.
Professor David Jones of St Mary’s University College, London, said: “People may well have ticked a box about medical research in hospital at some point in the past while they were undergoing tests and not even recall it now.”
Prof Jones, who is the Director of the Centre for Bioethics and Emerging Technologies at St Mary’s, added: “That tissue could be used to clone an embryo, and you would not even be told it was happening.”
The Human Fertilisation and Embryology Authority said there was only a “very small” chance of researchers using human tissue to create hybrid embryos without the donor’s consent.
The creation of animal-human embryos was permitted by the Human Fertilisation and Embryology Act 2008.
Before the Act was passed, an opinion poll found that 60 per cent of the public opposed the creation of animal-human embryos.
In February, it was reported that a new study had cast doubt on whether the hybrid embryos would ever lead to medical treatments.