Evan Harris will tonight spearhead a defence in Parliament of a woman's right to have an abortion up to 24 weeks into pregnancy.

The Oxford West and Abingdon MP will fight moves by pro-life campaigners to reduce the upper legal limit to 22 or even 20 weeks, on the grounds that there is "no good medical evidence" to support it.

The controversial issue will be debated in the form of amendments to the Human Fertilisation and Embryology Bill in the House of Commons.

Witney MP David Cameron and Wantage MP Ed Vaizey have both indicated they will vote for lower time limits because they believe scientific advances mean babies are now more likely to survive if delivered under 24 weeks.

But Dr Harris, the Liberal Democrats' science spokesman, told the Oxford Mail: "There is no good medical evidence to support any reduction in the current 24 week abortion time limit.

"Less than two per cent of abortions take place above 20 weeks' gestation and these involve the most vulnerable women in the most difficult personal circumstances who are not taking abortion lightly.

"Those who are against abortion should not punish this small group of women by denying them access to safe abortion but instead should support measures to reduce the number of unwanted pregnancies by improving sex and relationships education and improving access to effective contraception."

Dr Harris, one of the most vocal defenders of the right to abortion, is one of 86 MPs who have backed a Parliamentary early day motion that argues the current 24-week limit is "ethically and scientifically justified".

The motion warns that reducing the abortion time limit would "force some women to continue a pregnancy and give birth against their will".

It cites a recent report by the House of Commons Science and Technology Committee, which concluded that below 24 weeks it had seen "no good evidence to suggest that the foetal viability has improved significantly since the abortion time limit was last set, and seen some good evidence to suggest that it has not".