Geraldine Smith, MP for Morecambe and Lunesdale, was one of nine Labour MPs to rebel on Monday night against flagship measures to liberalise embryo research.

She told MPs she was defying the government with "sorrow", but said her office postbag proved that the issue "horrifies" a large number of her constituents.

The Human Fertilisation and Embryology Bill permits the creation of embryos containing human and animal material for studying diseases like Parkinson's and Alzheimer's.

The legislation would allow scientists to grow the embryos for up to 14 days.

Following opposition from Catholic bishops and MPs, Gordon Brown agreed to allow a series of free votes on the most controversial issues next week.

But all Labour MPs were expected to back the Bill on Monday night at its second Commons reading.

However, during an impassioned speech, Ms Smith said: "I want to challenge the provisions in the Bill that will add to the two million human embryos that since 1990 have been destroyed or experimented upon.

"I find this aspect of the Bill revolting and, if my postbag is anything to go by, it also horrifies a good number of my constituents.

"I have always had serious misgivings about the ethics of this issue, as well as a real concern about the lack of progress in embryonic stem cell research.

"It is time that the argument shifted to us putting resources into the new methods that have been developed to produce stem cells with greater potential than embryonic stem cells."

Ms Smith also spoke out against plans to allow "saviour siblings" - screened as embryos to ensure suitability to donate umbilical cord cells to sick children - and on removing the requirement that IVF clinics consider a child's need for a father, to give full rights to single women and lesbians.

She described the latter proposal as "one of the most ill-conceived measures to be put before the House while I have been a Member".

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