By Pallab Ghosh
Science correspondent, BBC News
Leading scientists have urged peers not to block the use of human-animal hybrid embryos for research.
Hybrids are made using an animal egg mixed with human genes
Amendments to the Human Fertilisation and Embryology Bill would restrict the use of such embryos, which contain a small amount of animal DNA.
But the scientists say they are vital to the development of new treatments.
The appeal comes from the Medical Research Council, the Royal Society, the Wellcome Trust and the Academy of Medical Sciences.
The government's bill attempts to update the current rules on the use of embryos for research in light of recent scientific advances.
One of the most contentious proposals is to enable researchers to create animal-human hybrids for research purposes - a move that some in the House of Lords are keen to overturn.
Supporters of inter-species research say it could one day help to treat serious diseases - such as cancer and HIV/Aids.
But opponents say the research is profoundly unethical.
Cross bench peer Lord Alton, a member of the all-party Parliamentary Pro-Life group, has proposed an amendment that would prohibit the creation of inter-species embryos.
If that fails Lord Alton's supporters want to toughen up the process of assessment by which licences for inter-species embryos are granted.
Josephine Quintavalle of the campaign group Comment on Reproductive Ethics (CORE) said whatever happened in the Lords, the battle against inter-species embryos would continue fiercely in the Commons.
"If total prohibition is not possible there, then at the very least we would like to see robust measures imposed, which would turn objective and scrupulous scrutiny of licence applications in this field into a convincing reality, rather than the nonsense we have to put up with currently."
Mrs Quintavalle said that under the current system, approval for embryo research had always been forthcoming from the Human Fertilisation and Embryology Authority.
On Tuesday, campaigners from the group Christian Concern For Our Nation demonstrated outside Parliament as the peers began their consideration of the Bill.
But in a briefing document, the four research institutions spell out why they see hybrid embryos as so important.
"This research has massive potential to provide treatments for serious debilitating disorders ranging from developmental abnormalities in young children, to stroke, cancer, HIV/Aids, diabetes and Parkinson's disease, as well as better and safer treatment for infertile couples.
"The UK's strengths in this field present valuable opportunities to influence the international agenda, drive the translation of basic research towards clinical benefits and attract skilled scientists and international investment in stem cell research to the UK."
The briefing document also calls for the hybrids to be known as "human admixed" embryos to reflect the fact that are overwhelmingly made of human tissue, with just a tiny amount of animal DNA added in.
The scientists want further changes to the Bill to "future proof" the legislation.
They are also backing an amendment to enable the regulator to allow doctors to offer stem cell therapies - if they are shown to be promising. At present the regulator can only license research.
But the briefing falls short of supporting an amendment by an obstetric expert, Lord Patel, that would enable doctors to use artificial sperm and eggs cloned from patients to be used in fertility treatments, if the procedure has been shown to be safe.
However, according to Lord Patel, his amendment does have strong support from the scientific and medical community.