On Tuesday, MPs have a further free vote on the emotive issue of cutting the abortion time limit.
Mr Cameron, along with Mr Brown, has backed the use of hybrid embryos as a means to develop treatments for cancer and conditions such as Parkinson's and Alzheimer's disease. They also both support the creation of "saviour siblings".
However, the majority of the Tory shadow cabinet, including shadow foreign secretary William Hague and shadow home secretary David Davis, backed the unsuccessful attempt to ban hybrids.
Ex-minister Edward Leigh, who led the fight against the creation of hybrid "admixed" embryos, said they were "ethically wrong and almost certainly medically useless".
He said there was "no evidence yet to substantiate" claims the work could lead to treatment for degenerative diseases like Parkinson's and Alzheimer's.
The bill would allow regulated research using hybrid or "admix" embryos, where the nuclei of human cells are inserted into animal eggs. The resulting embryos would be kept for up to 14 days to harvest stem cells.
Health Minister Dawn Primarolo says any research done using human embryos "must satisfy the Human Fertilisation and Embryology Authority that it was necessary or desirable".
No human "admix" embryo would be implanted into a woman or animal, she says.
'Too human'?
But Mr Leigh said: "We do not believe that regulation is enough. We believe this is a step too far and therefore should be banned.
"In embryos, we do have the genetic make up of a complete human being and we could not and should not be spliced together with the animal kingdom."
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