Gordon Brown today gave his unequivocal backing to the creation of human-animal embryos, insisting the controversial research could save millions of lives.

Just a day before MPs vote on the Human Fertility and Embryology Bill, the Prime Minister gave his most vociferous support yet and said developing “hybrid” techniques was an “inherently moral endeavour”.

He said MPs had a duty to themselves and “future generations” to permit the use of human-animal hybrids, which could be used to tackle diseases like Parkinson’s and Alzheimer’s.

Mr Brown said: “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.”

The Prime Minister, whose son Fraser suffers from cystic fibrosis, a condition that could benefit from the research, attacked critics of the Bill, who condemn the research as “Frankenstein science”.

“I have deep respect for those who do not agree with some of the provisions in the Bill because of religious conviction,” he wrote.

“But I believe that we owe it to ourselves and future generations to introduce these measures, and in particular, to give our unequivocal backing within the right framework of rules and standards, to stem cell research.”

Mr Brown said allowing the use of human-animal “admixed” embryos could remove the “critical limiting factor” on stem cell cures - a lack of human eggs from which to generate embryos.

“Let me be clear: if we want to sustain stem cell research and bring new cures and treatments to millions of people, I believe admixed embryos are necessary,” he said.

“The question for me is not whether they should exist, but how their use should be controlled.”

Mr Brown supports scientists who want to insert the nuclei of human cells into animal eggs, creating hybrid embryos which are allowed to grow for a few days.

Stem cells can then be harvested and used to create brain, skin, heart and other tissue for treating diseases - before the embryos are destroyed.

After a Government u-turn MPs now have free votes over the next two days on three contentious proposals: hybrid embryos; the creation of “saviour siblings” whose genetic material could help sick relatives; and giving lesbians equal rights to IVF treatment.

Three Catholic cabinet ministers - Ruth Kelly, Des Browne and Paul Murphy – are debating whether to vote against the Government.

Another three ministers have signalled they may vote to lower the upper time limit for so-called 'social abortions' from 24 weeks of pregnancy.

Mr Brown insisted Britain should not “turn our back” on vital scientific advances.

Is the Prime Minister right? Can the worlds of science and religion find common ground? Use the form below to give us your views.

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