As the Commons prepares for a crunch vote, the Prime Minister said developing "hybrid" techniques was an "inherently moral endeavour".

MPs owe it "to ourselves and future generations" to pass measures permitting the use of such embryos, he argued.

Mr Brown's comments, in an article for the Observer, have a personal dimension because his younger son, Fraser, has cystic fibrosis - a condition that could benefit from embryo research.

The premier 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.

Critics of the hybrid technology have branded it "Frankenstein science". But 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."

MPs have free votes over the next two days on three controversial proposals: hybrid embryos; the creation of "saviour siblings" whose genetic material could help sick relatives; and giving lesbians equal rights to IVF treatment.

The Human Fertility and Embryology Bill will also see free votes on amendments to tighten abortion laws, by reducing the upper limit from 24 weeks to 22 or even 20 weeks.

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