The Prime Minister has accepted that he will have to sacrifice a measure on parenthood in order to save legislation to allow new embryo research and treatments.

Labour MPs determined to oppose the Human Fertilisation and Embryology Bill are being quietly urged to direct their anger at proposals to scrap the requirement that IVF clinics consider the child’s “need for a father”, The Times has learnt. Equality campaigners say that the requirement breaches the human rights of lesbians seeking fertility treatment. But ministers believe that the measure is marginal compared with tonight’s key votes on allowing the creation of human-animal hybrid embyros and so-called saviour siblings.

Over the weekend Gordon Brown wrote that human-animal embryo research could save millions of lives. Dismissing critics who say that the technology amounts to “Frankenstein science”, he said that the technique would reduce reliance on human eggs, “the critical limiting factor” holding back stem-cell research. “The scientists I speak to are committed to what they see as an inherently moral endeavour that can save and improve the lives of thousands and over time, millions.” Mr Brown had to give Labour MPs a series of free votes during the Bill’s committee stage after it became clear that his party and Cabinet were divided on its three most contentious clauses.

MPs will also vote according to conscience tomorrow on amendments to reduce the abortion limit from its 24-week maximum to 22 weeks or even 20 weeks.

The Conservatives are encouraging their MPs to vote against the Government’s proposal to remove the stipulation that IVF clinics consider the “need for a father”. A growing number of Labour MPs are considering voting against the measure tomorrow.

For now, Mr Brown and Alan Johnson, the Health Secretary, are seeking to stave off defeat tonight. Research charities last night urged MPs not to ban so-called “true” hybid embryos made up of 50 per cent human and 50 per cent animal DNA. Andrew Lansley, the Shadow Health Secretary, has tabled an amendment outlawing them.

The Wellcome Trust and the Medical Research Council said in a letter to MPs that such a ban “would prevent research to improve our understanding of infertility, sperm function, early embryo and stem-cell development”.

Stem-cell treatments for diseases such as Parkinson’s and diabetes could still become illegal. Though the Bill is intended to enable research into these therapies, it contains no mechanism allowing them to be given to patients should the work prove successful. The problem with the legislation, first reported by The Times in March, has been repeatedly raised with ministers by scientists, lawyers and patient groups, but the Government has refused to address it.

An amendment tabled by Evan Harris, the Liberal Democrat science spokesman, that would resolve the problem, will be debated by MPs today. It is supported by Mr Lansley. Dr Harris said yesterday: “It is essential that there is no doubt that the law permits embryos to be created, stored and used purely for treating patients.”

Embryonic stem cells are master cells found in human embryos, which can grow into any type of tissue. They hold great promise for treating conditions such as diabetes, Parkinson’s and spinal paralysis. The Bill allows the Human Fertilisation and Embryology Authority (HFEA) to authorise the use of human embryos only for medical research or for treating infertile patients.

Embryonic cell therapies are not yet ready to be given to patients, and their first applications in clinical trials would be covered by the research provision. If techniques move beyond an experimental stage, however, biotechnology companies would be forbidden from generating stem cells from embryos purely to use in treatment, were no research involved.

The Health Department has stated that there is not an issue, because embryonic stem cells created by research projects are no longer licensed by the HFEA, and could be approved for therapy by other agencies. Yet officials have admitted privately that the new law would not permit the use of embryos for purely therapeutic purposes.

The Bill’s clauses that allow experiments with human-animal embryos have been backed by research charities and patient groups, eight of which have written to MPs to urge them to vote for this research. It could “fast-track” the development of new treatments, they said. “The overwhelming majority of those affected by conditions such as Parkinson’s, Alzheimer’s, motor neurone disease, multiple sclerosis, diabetes, cystic fibrosis and muscular dystrophy do not want any stone to be left unturned in the search for more effective treatments and cures.”

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