Gordon Brown argues that the Human Fertilisation and Embryology Bill is needed to enable vital medical research into treatment for conditions such as Parkinson's, Alzheimer's, cancer and heart disease.

But some of the provisions in the Bill - most notably the creation of hybrid or "admix" embryos through the combination of animal eggs and human nuclei - have proved highly controversial.

At the same time, the Bill will seek a fresh attempt to lower the current 24 week time limit on abortions in what is expected to be a fiercely contested parliamentary battle on both sides of the debate.

However, a poll of 109 MPs on Monday by The Guardian suggests that while extending embryo research would succeed, moves to change the abortion law would be defeated.

Fifty-seven said that they would vote to keep the existing time limit, while 36 would vote for change, with two abstentions and 14 undecided.

Nevertheless, the Prime Minister has already been forced to grant Labour MPs a free vote on key elements of the legislation after a threatened revolt by Roman Catholic ministers, including three members of the the Cabinet.

But even that has not satisfied some Labour MPs who say they should have the right to vote with their consciences on the whole of the legislation, as the Tories and Liberal Democrats have been promised.

The Bill has been strongly criticised by some church leaders, most notably the leader of the Roman Catholic Church in Scotland, Cardinal Keith O'Brien, who denounced it as a "monstrous" attack on human rights and dignity.

In what has been an increasingly acrimonious debate, researchers have in turn accused the clergy of wilfully misunderstanding the science involved. They point out that hybrid embryos would not be implanted in women or animals to create offspring, but would be used to harvest stem cells for research before being destroyed.