Opponents say the bill will lead to 'Frankenstein'-style experiments

Fertility expert Lord Winston said statements made by Cardinal Keith O'Brien were discrediting the Church.

Some 200 medical charities have urged MPs to support legislation allowing the creation of animal-human embryos.

Scientists say hybrid embryos will bring medical benefits but detractors in the Church say it is immoral.

Shortage of eggs

The controversy first began when the government announced plans to update the Human Fertilisation and Embryology Act, which would expand the scope of embryo research.

The legislation would allow scientists to combine animal eggs with human nuclei, which can then be grown into stem cells.

It was introduced because of a shortage of donated human eggs.

Over the Easter weekend, a number of Catholic Church leaders weighed into the dispute, saying the plans undermined the sanctity of life.

I have huge respect for the Catholic Church...but it will be destroying its probity with overblown statements of this kind

Lord Winston

In his Easter sermon, Cardinal O'Brien described the legislation as a "monstrous attack on human rights, human dignity and human life", adding that it would allow experiments of "Frankenstein proportion".

But Lord Winston told the Daily Telegraph he thought the cardinal's statements were inaccurate.

He said: "They are misleading and I'm afraid that when the Church, for good motives, tells untruths, it brings discredit upon itself."

"I have huge respect for the Catholic Church, which does great good, but it will be destroying its probity with overblown statements of this kind."

The row has intensified with a number of Catholic Labour cabinet members warning that they may vote against the government.

Claims defended

Catholic leaders have denied misleading the public over the issue.

The Archbishop of Cardiff, Peter Smith, called for a "calm, reasoned debate based on the facts".

He said: "I refute any suggestion that the Catholic bishops have been lying about this Bill or misleading people as to its contents and import," he said.

"Contrary to what a number of scientists have claimed, this Bill does in fact allow the creation of animal-human hybrid embryos for research."

The Bishop of Lichfield, the Rt Revd Jonathan Gledhill, said: "It's a very important part of our society and a very important part of the Christian faith that you should have respect for human embryos.

"If you stop obeying God you start to limit the rights of human beings and this is a case in point."

Research 'hampered'

Cancer Research and the British Heart Foundation are among more than 200 charities in favour of the creation of human-animal hybrids for research.

In a letter to every MP, the charities argue that current research is being "significantly hampered" by a shortage of donated human eggs.

The charities said: "The bill will allow new avenues of scientific inquiry to be pursued which could greatly increase our understanding of serious medical conditions affecting millions of people throughout the UK."

They are hopeful of finding cures for diseases and genetic disorders such as cancer, heart disease, Parkinson's and muscular dystrophy.

Free vote

The government appears to be seeking a compromise over the bill after warnings that substantial number of Labour MPs are ready to defy the government.

Health Secretary Alan Johnson said the government would seek an "accommodation" with Labour MPs allowing them to vote according to their conscience rather than on party lines.

Tory leader David Cameron has urged Gordon Brown to allow all MPs a free vote on the issue.

He told BBC London radio: "What he should say is 'this is an important bill... it does raise issues of conscience, so there should be free votes throughout the bill's passage'."

The bill received its first reading in the House of Commons in February, and while no date has been set for its second reading - or approval in principle - it is viewed as a key piece of legislation on the government's agenda.