Gordon Brown has said that Labour MPs will be given a free vote on certain elements of the Human Fertilisation and Embryology Bill.

The legislation includes plans to create human-animal hybrid embryos which the government argues is crucial to advancing scientific research into diseases such as Parkinson's and Alzheimer's.

However, Catholic MPs and ministers are thought to have raised ethical concerns over the move and the compromise comes following warnings that Brown could have been faced with a backbench rebellion on the issue.

The prime minister told reporters on Tuesday that backbenchers would be allowed to defy the whip on three controversial sections of the legislation - IVF research, saviour siblings and Admix embryo.

Catholic MPs and ministers are understood to be particularly keen to avoid supporting sections of the Bill on allowing scientists to create embryos which combine human DNA and animal cells, which the government argues is crucial to advancing scientific research.

The Conservatives and Liberal Democrats have said they will grant a free vote, and former Labour minister Stephen Byers said on Sunday the public would "look on in disbelief" if Brown failed to do so.

Tory leader David Cameron has cautioned the Catholic church to moderate its criticism of the legislation, using the example of his own son's suffering from a neurological condition to call for a "frank and realistic debate".

"There is a danger that people can overstate what is in this Bill and that is all the greater need for it to be debated calmly and reasonably in Parliament," he said.