London:  Prime Minister Gordon Brown on Wednesday promised Labour MPs a free vote on the most emotive measures in the embryo research Bill.

In a compromise to avoid splitting his Cabinet, the prime minister said Catholic MPs would be able to vote with their "conscience" - rather than simply abstain as he had indicated previously.

His major concession comes after up to a dozen ministers, including three Cabinet members, signalled they would resign rather than obey a three-line whip. Pressure intensified on Brown when leaders of the major faiths demanded MPs be given a free vote.

Faced with a damaging revolt, Brown said that MPs would have a free vote on three issues - research using part-human, part-animal embryos, saviour siblings and advanced fertility treatment.

But he insisted that all other aspects of the Bill would be whipped and he expected his MPs to vote for the whole package of legislation at the second and third readings. "We will respect the conscience of every member of parliament as they decide how to cast their vote," Brown said.

At the weekend cardinal Keith O'Brien the Catholic leader in Scotland, used his Easter Day sermon to condemn the "monstrous" proposals.

A spokesman for him said yesterday: "Even with a limited free vote there are still many troubling aspects of this Bill which could be passed into law."

Not enough

The Human Fertilisation and Embryology Bill is aimed at bringing existing laws on fertility treatment and embryo research in line with scientific advances. But Brown's announcement was not enough to win round all dissenting MPs and raised doubts about whether his party would fall into line at the second and third readings.

Crosby MP Claire Curtis-Thomas said that at the "final hurdle" of the third reading she would still have to vote with her conscience regardless of whether she had a free vote on individual parts of the Bill.

"This is a primarily moral question and at the end of the day I have to answer to my conscience and whatever stage this arises I would not able to agree to it."