The Bill has split the Cabinet and set off a war of words between senior Labour figures and the Roman Catholic Church.

There are two aspects of the sensitive Bill that have raised a storm. One is that it would allow homosexual couples to be registered as legal parents. The other, more contentious clause is the one allowing for the creation of human-animal hybrids.

Critics say using human-animal embryos for experiments would be ethically unacceptable. Researchers can create hybrid embryos by injecting human DNA into a hollowed-out animal egg cell.

A succession of senior Catholic Church leaders used their Easter addresses on Sunday to criticise the legislation and intensify the pressure on Prime Minister Brown to allow his Catholic MPs a free vote when the House of Commons debates the Bill later this year.

Cardinal Keith O'Brien, the head of the Catholic Church in Scotland, used his sermon to attack the proposals, claiming they would allow Frankenstein-style experiments.

Cardinal Cormac Murphy-O'Connor, the head of the Catholic Church in England and Wales, said Catholic legislators should be able to vote according to their conscience.

Senior Labour MPs and peers, in turn, have accused the Catholic Church of trying to 'dictate government policy' and of 'scaremongering', said a report in the Telegraph yesterday.

Mr Brown is hoping to pass the new laws, and may order party members to back the plans.

But three senior members of his Cabinet - Defence Secretary Des Browne, Transport Secretary Ruth Kelly and Welsh Secretary Paul Murphy - are Catholics.

And if Mr Brown imposes voting rules on his party, all three would be expected to resign if they oppose the plans.

With five million Catholics in Britain, such a public row risks causing significant political damage to Labour, when the party is languishing in the polls, said the Telegraph report.

Mr Stephen Byers, a former Labour Cabinet minister, warned at the weekend that the government risked a backlash from voters if it did not agree to a free vote.

'The public will look on in disbelief if a matter as sensitive as the creation of human-animal embryos is made a matter of party policy, with the government instructing its MPs how to vote,' he said.

The Bill was part of Mr Brown's first Queen's Speech. It is considered a flagship Bill that would help push the boundaries of medical research.

Ministers say Britain's Human Fertilisation and Embryology Bill, which was last updated in 1990, must be redrawn to take account of scientific advances.

Peers in the House of Lords have already rejected an attempt to remove proposals related to hybrid human-animal embryos from the Bill. An amendment was defeated by 268 votes to 96 in a vote last month.

The government has not yet set a date for a Commons vote on the proposed laws but it is confident it can push through the legislation.

'I'm convinced the House of Commons will carry this Bill,' Health Secretary Alan Johnson told Sky News on Sunday. 'There will not be a Cabinet split, but there will be an accommodation for those who have a particular sensitivity around this.'

Britain is one of the leading states for stem cell research, attracting scientists from around the world with a permissive environment that allows embryo studies within strict guidelines.

ASSOCIATED PRESS, REUTERS