Plot to force abortion vote

They want the cut-off for terminations to come down from 24 weeks to 21 or 20 weeks, as medical advances mean some premature babies survive earlier births.

If the MPs succeed in forcing ministers to hold a vote on the abortion limit, it would be the first on the issue since 1990, when it was reduced from 28 weeks.

Health Minister Caroline Flint revealed the manoeuvring in a leaked memo to Tony Blair.

She warned the Prime Minister that her forthcoming draft Human Tissue and Embryos Bill, which would change the law on fertilisation and embryology, could be used to raise the subject.

The memo read:

"There is a possibility that some members may wish to use the opportunity presented by the draft Bill to discuss wider issues dealt with by the original legislation. . . and related topics of interest, notably abortion (under the Abortion Act 1967).

"Advice from the House authorities suggests that these topics could not simply be ruled out as a matter of scope."

Conservative MP Ann Widdecombe said the opportunity would not be missed.

She said: "We will go for it and we will go hard and fast on it. I think it would be a major campaign and I would absolutely be there."

Any vote would almost certainly see MPs freed from the party whip and allowed to vote according to conscience.

But the Marie Stopes organisation, which carries out a third of the 181,000 abortions in England and Wales each year, said terminations at 20 or more weeks gestation were "rare" - making up about 3,000, or 1.6 per cent.

Spokesman Tony Kerridge said: "These MPs seem to be suggesting that women wait deliberately until they are in the later stages of pregnancy and then almost flippantly decide that they want an abortion.

"No woman acts in that way having been pregnant for nearly six months. For most, there have been delays in the system or their personal circumstances have changed dramatically."

Abortion is legal in Britain up to full term if doctors believe the baby has a severe disability or the mother's life is at risk.

But termination for what are termed "social" reasons - including the effect of pregnancy on the mother's mental health and wellbeing - is legal only up to 24 weeks.