Gordon Brown will vote against the reduction in the upper time limit for abortion in next week's Commons debate on the controversial Human Fertilisation and Embryology Bill.
The Prime Minister's lead on the Bill, backed by a majority of Labour women, means that calls for a reduction will almost certainly be defeated.
A cross-party group of MPs, backed by the Pro-Life group, are calling for the legal time limit for abortion to be reduced from 24 weeks to 20 weeks or lower, claiming that medical advances have led to babies surviving at an earlier age.
The senior Tory MP Edward Leigh has tabled an amendment to the Bill calling for the upper time limit to be reduced to 12 weeks.
Mr Brown accepted a report by the BMA, which is strongly disputed by the Pro-Life campaigners, that there has been no improvement in the survival rate under 24 weeks.
There will also be a battle over fertility treatment for lesbians and advice by the Human Fertilisation and Embryology Authority, including its assessment that a child "needs" a father.
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