The Leader of the Opposition has said that critics of new legislation should be careful not to “overstate” its impact.

David Cameron’s comments follow an Easter weekend full of claim and counter-claim about the creation of “animal-human hybrids” as a result of the Human Fertilisation and Embryology Bill.

On Easter Sunday the leader of Scotland’s Roman Catholics attacked the Prime Minister and made lurid claims that the new legislation “comprehensively attacks the sanctity and dignity of human life than this particular bill.”

“Gordon Brown is promoting a bill that will allow the creation of animal-human hybrid embryos,” said Cardinal Keith O’Brien.

More than 200 charities have written a joint letter to all MPs.

“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 said.

The respected peer and fertility expert Lord Winston accused the Roman Catholic leader of “misleading” the public about the nature of the science involved.

“I’m afraid that when the Church, for good motives, tells untruths, it brings discredit upon itself,” he said.

Today’s warning from Mr Cameron stressed that the present atmosphere may prevent the legislation from being debated “calmly and reasonably” by MPs.

“My own view, and I think [that of] many people in the Conservative Party, is we need to update the legislation,” he told GMTV.

“This sort of research is important. We all want to see diseases reduced and problems that children have, birth defects, dealt with.”

Initially the Prime Minister rejected a free vote on the bill, which the Tories and Lib Dems have granted to their MPs, regarding the legislation as a matter of conscience.

However, it is thought likely that Labour MPs may be able to opt out of approving the bill but it is unclear what position Roman Catholic Cabinet ministers such as Ruth Kelly will be asked to take.

The Human Fertilisation and Embryology Bill proposes new recognition of same-sex couples as legal parents of children conceived through the use of donated sperm, eggs or embryos.

A woman who gives birth and her civil partner will both be recognised as the parents of a child conceived through assisted reproduction.

Two men will be able to apply for a parental order to become parents of a child conceived through a surrogacy arrangement.

Other church leaders have backed Cardinal O’Brien.

The Archbishop of Cardiff, Peter Smith, told the BBC:

“I refute any suggestion that the Catholic bishops have been lying about this Bill or misleading people as to its contents and import.

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

It is not clear when the bill will come before the Commons.