The Catholics expected to resign from cabinet over fertility vote

A senior Catholic MP told the Daily Mail that more than one minister was ready to consider their position over the issue because of their faith.

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Discussions between those opposed to the Human Fertilisation and Embryology Bill and Geoff Hoon, the Chief Whip, are understood to have resulted in stalemate and soured Cabinet relations.

The legislation will permit lesbian couples to be registered as legal parents for the first time, allow the creation of human-animal hybrid embryos for scientific research and remove the requirement on IVF clinics to consider a child's need for a father.

It will also give the green light to the creation of "saviour siblings" - children genetically selected to be a donor for a sick brother or sister.

David Cameron has said Tory MPs will get a free vote on all aspects of the Bill and the Liberal Democrats are expected to follow suit for its most controversial elements.

The last shake-up of fertility laws, in 1990, was subject to a free vote under the then Tory government. But so far Labour has refused to follow tradition on issues of conscience and allow MPs to vote as they wish on the legislation.

Instead, they face a whipped vote, meaning they could face disciplinary action if they refuse to support it.

During the Bill's passage through the Lords, the Government imposed a three-line whip on Labour peers for every vote.

Mr Hoon has offered MPs the option of writing to him asking to be excused from the vote on religious or ethical grounds. But he has made it clear that they must not vote against.

Ministers understood to be most unhappy about his stance are Welsh Secretary Paul Murphy, Transport Secretary Ruth Kelly and Defence Secretary Des Browne, all of whom are Catholics. If any of them defied a three-line whip they would be expected to resign.

"The ministers opposed to the Bill haven't made any progress at all," said a source close the Government.

Mr Murphy is said to have told colleagues he is so angry about the Bill that he is prepared to defy the Government for the first time in his career.

Opus Dei, the orthodox Roman Catholic group of which Miss Kelly is a member, has declared that without a free vote it is "practically impossible to be a Labour MP and a practising Catholic".

Three Government whips - Tommy McAvoy, Tony Cunningham and Frank Roy, all Catholics - also have serious ethical problems with some of the Bill.

Labour MP Jim Dobbin, chairman of the all-party Parliamentary prolife group, is due to meet the Prime Minister with pro-life colleagues to make a final appeal for a free vote.

He said there would be "considerable disruption across the Labour Party" if one was not conceded. "A lot of people who do not come from any particular religious or moral standpoint are voicing concern about the removal of the need for a father clause," he said.

"I'm not going to put any minister on the spot at this stage but I do think some of them will consider their positions. Even if there is not a free vote I will be voting against this Bill. There is nothing party political at all about destructive experiments on human embryos."

Mr Brown told MPs this week the "important Bill improves the facilities for research and is vital for dealing with life-threatening diseases".

He said all MPs would have the right to follow their "consciences".

But a source close to Mr Hoon insisted: "You can exercise your conscience by saying 'I'm sorry, it's a Government Bill but I can't vote for it'. It is vital that we pass this Bill.

The legislation will also provide an opportunity for the first amendments by MPs to the 1967 Abortion Act in 18 years. Some MPs want to lower the upper time limit from 24 weeks but others are preparing a raft of amendments to further liberalise the law.