Gordon Brown and David Cameron have both firmly backed the ground-breaking technique as a means to develop treatments for common conditions that could potentially save millions of lives.

The Prime Minister and Tory leader also support the creation of "saviour siblings" selected by parents in order to provide tissue material for seriously ill children.

Mr Brown has hailed both innovations as "an inherently moral endeavour". But opponents from all parties hope to unite to axe both from the Human Fertilisation and Embryology Bill in crunch Commons votes on Monday evening.

And with MPs allowed to vote according to personal conscience, not the party line, on the most contentious elements of the Bill, the result is not predictable.

Leading the fight against so-called "admixed embryos", Tory MP Edward Leigh said the technique, which involves implanting a human nucleus inside an animal cell, was "a step too far".

Scientists believe that stem cells harvested from these embryos could provide the key to breakthroughs in the treatment of conditions such as Parkinson's disease, cystic fibrosis or muscular dystrophy. Research is currently being held up by the shortage of human eggs to create stem cells.

But critics of hybrids have branded them "Frankenstein science", saying it is obscene to combine human and animal genetics. They also question the benefits, insisting other methods are more effective.

But Mr Leigh said 21 other countries had banned the creation of hybrids. "In terms of embryonic research we will almost be like a rogue state," he warned.

But Labour MP Chris Bryant, a former Anglican curate, compared Mr Leigh's arguments to those used by church leaders against the smallpox vaccine. He said: "They were wrong and I think you are wrong today."