The leader of the Roman Catholic church in Scotland has urged the prime minister to rethink "monstrous" plans to allow hybrid human-animal embryos.
Cardinal Keith O'Brien will use his Easter Sunday sermon to launch an attack on the government's proposals.
He will also call on Gordon Brown to allow Labour MPs a free vote on the issue at Westminster.
Downing Street did not respond directly to the cardinal's attack, saying Mr Brown had already made his views clear.
The prime minister has said the bill would improve research into many illnesses.
Supporters of the bill believe hybrid embryos could lead to cures for diseases including multiple sclerosis and Alzheimer's disease.
Leading scientists accused the Roman Catholic Church of "scaremongering" over research which had the potential to save many lives.
Cardinal O'Brien, who is the Archbishop of St Andrews and Edinburgh, also wants Catholic ministers serving in the Cabinet to stand down rather than support the bill.
In his sermon, which was released on Friday, the cardinal claims that the bill would lead to the endorsement of experiments of "Frankenstein proportions".
He says: "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."
The cardinal describes the practice as "grotesque" and "hideous".
Dr Stephen Minger, director of the stem cell biology laboratory at King's College London, said: "This is yet another example where it is clear that the Catholic Church is misrepresenting science because it doesn't understand the basic facts."
He added: "The church should carefully review the science they are commenting on, and ensure that their official comments are accurate, before seriously misinforming their congregations."
A Department of Health spokeswoman said: "The Government listened very carefully to arguments from the scientific community about the need to create 'hybrid' embryos.
"This matter has been closely scrutinised by parliamentary select committees, and voted on by the House of Lords.
"The Human Fertilisation and Embryology Authority also ran a public consultation on this issue last year."
The spokeswoman said the Government was proposing "strict controls" on the research which were broadly the same as those applying to research on human embryos.
"There will be a limit of 14 days' development of the embryo, and they cannot be put in a woman or an animal," she added.
"This is not about 'creating monsters'. It is purely laboratory research, and is aimed at increasing knowledge about serious diseases and treatments for them."
In his sermon, Cardinal O'Brien also calls for the establishment of a "single permanent national bioethics commission".
He has written to Mr Brown to tell him that this would be the only way in which the issue could be "adequately discussed".
A proposed amendment to the bill, which would have prohibited the creation of inter-species embryos - known as human admixed embryos - was defeated by 268 votes to 96 in the House of Lords in January.
Labour peers were instructed to follow the party whip by voting against the proposed amendment.
HAVE YOUR SAY
I wonder how the cardinal would view it if one of his own family were to benefit from any future success in this field
James Rooney, Glasgow
Conservative leader David Cameron has called on Mr Brown to allow Labour MPs to have a free vote on the bill when it returns to the House of Commons later in the year.
It has been reported that a Labour MP said Welsh Secretary Paul Murphy would resign rather than vote for the Embryology Bill.
A spokesman for Mr Murphy denied that he would go that far but said there were "big scientific and conscience issues involved".
Mr Murphy's spokesman said everyone should have the right to exercise their conscience and said that ministers were "trying to find a way through it".
Scottish National Party Westminster leader Angus Robertson also urged Mr Brown to allow Labour MPs to vote according to their beliefs.
Mr Robertson said: "It is essential that MPs are allowed a free vote on this issue so they are not forced to compromise their own beliefs.
"There will be no whip on SNP MPs, and I would encourage Gordon Brown to allow Labour MPs the same right to vote according to their individual beliefs and moral judgment, rather than following a party whip."
Lib Dem MP Dr Evan Harris said the bill was a "decent and civilising" measure.
He said: "The use of terms like "monstrous" and "Frankenstein" to describe microscopic embryonic entities which contain animal and human material is preposterous scare-mongering given the millions of people who have received life-saving pig-heart valves.
"He is entitled to reject any treatment coming from this research on behalf of himself and his more devout followers but the millions of people hoping for medical research breakthroughs using stem cell technology would regard his attempt to veto this for them as well to be 'monstrous'."
Free votes allow MPs to vote according to their own beliefs rather than following the party whip.
Mr Brown has said a decision on whether a free vote will be held will be taken "in due course".