Labour MPs will be allowed a free vote on some controversial aspects of new embryology laws, Gordon Brown said today.
But he insisted they would be expected to support the Human Fertilisation and Embryology Bill as a whole and not block its progress.
The Prime Minister had been under increasing pressure from church leaders opposed to the proposals to allow MPs to follow their consciences.
He faced a rebellion of up to 12 Catholic ministers, including three members of the Cabinet.
Setting out a compromise solution, he said: "I have always said that although I attach huge importance to this legislation - in saving lives and helping to cure and treat diseases - that we will respect the conscience of every Member of Parliament as they decide how to cast their vote in this.
"On the three issues where, for the first time, these ethical issues are being debated in Parliament in this new way - and that's so-called admix embryos, the second one is saviour siblings and the third one is IVF research - exercising your conscience will mean for Labour Party members a free vote.
"But the Bill itself cannot be subject to a free vote because there are so many other changes that I believe are necessary as part of building up the research framework of our country and, of course, creating the right ethical framework for the development of embryo research."