One of the biggest political battles of recent years will move to the Commons today, with pro-life MPs confident they can defeat plans to extend embryo research.

One of the biggest political battles of recent years will move to the Commons today, with pro-life MPs confident they can defeat plans to extend embryo research while they muster votes for an attempt to lower the limit at which it is legal to have an abortion in the UK.

The second reading of the Human Fertilisation and Embryology Bill will herald the opening of a clash of morals which will come to a head in three weeks time when MPs discuss the most controversial measures.

During today's reading of the bill, anti-abortion campaigners will attempt to persuade the Commons to lower the legal cut-off point for most abortions from 24 to 20 weeks - arguing medical science has rendered the later limit out-of-date.

An alliance of Roman Catholic MPs, traditionalist Conservatives and representatives of other faiths are confident they will defeat plans to extend embryo research and stave off other scientific advances contained in the bill.

Provisions for the creation of human-animal embryos have been attacked by Roman Catholic bishops led by Scotland's Cardinal Keith O'Brien, who said the bill would allow "grotesque procedures" of "Frankenstein proportions".

Patient groups and scientists have accused these critics of trying to block life-saving research. Plans to remove the requirement that clinics consider a child's need for a father before starting IVF are equally contentious.

Gordon Brown has been forced to grant Labour MPs a free vote on some aspects of the bill in the face of a rebellion from Catholics within the party, including at least three cabinet ministers.

As well as the traditional conscience vote on abortion, MPs will not be whipped on amendments relating to the creation of animal/human hybrid embryos, so called "saviour siblings", and the scrapping of the requirement for a father during IVF treatment.

The all-party parliamentary pro-life group is confident as many as 60 Labour MPs will vote against all three measures, but yesterday the group wrote to the PM urging him to grant a free vote on every aspect of the contentious bill. Labour MPs are required to vote in line with government policy at the second and third reading of the bill, irrespective of their personally-held views, while other parties have given members a free vote.

"This requirement puts Labour MPs in an untenable position and in direct conflict with their own government," said MP Jim Dobbin, chairman of the all-party parliamentary pro-life group, in his letter urging Mr Brown to reconsider.

"Many members of parliament, including those who intend to vote with the government, are incapable of understanding why the Prime Minister is intent on this disastrous course of action, because no amount of persuasion, whipping or sanction will make some MPs vote against their conscience."