One of Britain's leading Catholics has attacked Gordon Brown over his Government's "monstrous" plans for embryo research and compared them to the creation of Frankenstein's monster.
The Human Fertilisation and Embryology Bill, which will allow scientists to create part human, part-animal embryos for use in stem cell experiments.
Cardinal O'Brien's attack on the Bill follows criticism of it from other Catholic leaders
His attack comes as reports suggest several Catholic Cabinet ministers are considering resigning because Mr Brown is refusing to allow them a free vote on the legislation, meaning they face disciplinary action if they refuse to support it.
Catholic leaders in England and Wales have already denounced the plans but the attack by Cardinal O Brien, the Church's leader in Scotland, is the most vitriolic to date.
Supporters of the legislation believe hybrid embryos could lead to cures for diseases including multiple sclerosis and Alzheimer's disease.
But in Sunday's sermon, Cardinal O'Brien will say: "This Bill represents a monstrous attack on human rights, human dignity and human life.
"In some European countries one could be jailed for doing what we intend to make legal.
"I can say that the Government has no mandate for these changes: they were not in any election manifesto, nor do they enjoy widespread public support."
Cardinal O'Brien will condemn Mr Brown for imposing a three-line whip on Labour MPs over the Bill and will also claim it would lead to the endorsement of experiments of "Frankenstein proportion".
His attack comes just days after the Archbishop of Cardiff, the Most Rev Peter Smith, condemned the proposals as "gross interference in human life".
The leader of the Roman Catholic Church in England and Wales, Cardinal Cormac Murphy-O'Connor, has also urged Catholics to write to their MPs to demand free votes because the Bill deals with "fundamental issues of personal conscience".
David Cameron has said Tory MPs will get a free vote on all aspects of the legislation and the Liberal Democrats are expected to follow suit for its most controversial elements.
But discussions between Labour MPs opposed to the Bill and Geoff Hoon, the party's Chief Whip, are understood to have resulted in a stalemate.
Mr Hoon has offered them the option of writing to him asking him to be excused from the vote on ethical grounds, but he has made clear they must not vote against.
Reports have said more than one Catholic minister is considering resigning from the Cabinet over the issue because of their faith.
The ministers understood to be most unhappy at being denied a free vote are Defence and Scottish Secretary Des Browne, Welsh Secretary Paul Murphy and Transport Secretary Ruth Kelly.
The Department of Health said the legislation has been scrutinised by parliamentary select committees and the House of Lords and been the subject of a public consultation.
A spokeswoman said: "The Government listened very carefully to arguments from the scientific community about the need to create 'hybrid' embryos.
"This is not about 'creating monsters'. It is purely laboratory research, and is aimed at increasing knowledge about serious diseases and treatments for them."
