MPs are voting on Tuesday on a proposed change to the law on abortion spearheaded by the Mid Befordshire Conservative MP Nadine Dorries.

The free vote represents the culmination of an 18-month campaign by the former nurse to lower the upper limit for abortions from 24 weeks to 20 weeks.

Mrs Dorries said: "The last 18 months have not been easy, but at least on Tuesday MPs will have the opportunity to vote on my amendment to reduce the upper limit for abortions from its current 24 weeks to 20 weeks."

She said it is "the final push in what has been a hard fought and focussed campaign to reduce the current upper limit".

In England and Wales there were 193,700 abortions last year - a rate of 18.3 per 1,000 women aged 15 to 44, higher than any other western European country.

Mrs Dorries believes that a cut in the upper time limit for abortions from 24 weeks to 20 would stop 2,500 terminations a year taking place.

At present around 200 MPs are behind the campaign, but they will only be able to overturn the law if at least 200 others abstain.

Over the past few months doubts have been raised over some of the facts used by Mrs Dorries to gain support for the issue.

Two weeks ago Ann Keen, junior health minister, told MPs that only 12 per cent of babies born below 24 weeks survived beyond their first birthday.

Lib Dem MP Evan Harris, a former hospital doctor, added: "The best research, which looks at every birth rather than a selected sample ... failed to show any reduction since 1995 in the threshold of viability below 24 weeks."

In March the Times & Citizen reported that a photo being used on Mrs Dorries blog to gain support was actually a well-known internet hoax.