Brighton MPs will tomorrow try to block an attempt in Parliament to remove a woman's right to an abortion up to 24 weeks after conception.

David Lepper (Pavilion) and Des Turner (Kemptown) will oppose moves by pro-life campaigners to reduce the upper legal limit to 22 or even 20 weeks, on the grounds that the medical evidence does not support such a change.

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

Several MPs - including Eastbourne's Nigel Waterson - plan to vote for lower time limits because they believe scientific advances mean babies are now more likely to survive if delivered under 24 weeks.

But Mr Lepper said: "I have not seen any reliable evidence that suggests there is greater viability for foetuses or babies at 20 to 22 weeks.

"There are always situations that arise where because of a whole range of circumstances women are only in a position to make a decision to have an abortion at the very last moment. That option should be available to them."

Dr Turner said a review of the scientific evidence by the cross-party Commons science committee, of which he is a member, found the survival rate of babies born before 24 weeks had not improved.

There was therefore "no basis in science or medicine to justify reducing the present time limit", which was itself cut from 28 weeks in 1990.

The MP said: "A woman's right to choose has been hard fought for and those who effectively would like to abolish abortion altogether are endeavouring to chop away at it by reducing the time limit."

Accusing some pro-lifers of "trying to undermine the very existence of abortion", he added: "They are certainly misrepresenting the available evidence".

Dr Turner is among 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 time limit would "force some women to continue a pregnancy and give birth against their will".

Hove MP Celia Barlow said she was "inclined" to vote to reduce the limit by a fortnight, to 22 weeks, because "a number" of foetuses survived at 23 weeks. But she added she had not yet made a final decision about how she would use her free vote.