Despite assurances from Health Secretary Alan Johnson that it would be subject to a "strict legal and ethical framework", some Labour MPs vowed to fight the measures.

And Labour came under renewed fire for not copying other parties by treating the Human Fertilisation and Embryology Bill as a conscience issue and giving its backbenchers a free vote.

Gordon Brown has already been forced to grant a free vote on the most contentious elements after a threatened revolt by Roman Catholic Ministers, including three Cabinet members.

Hybrid embryos, the creation of "saviour siblings" and ending the requirement for IVF clinics to consider the need for a "father" figure could be removed if enough MPs oppose them next week.

But the Prime Minister insists the Bill's overall potential to speed up research into treatment for conditions such as Parkinson's, Alzheimer's, cancer and heart disease means it must be passed.

"He thinks that it's important that a legislative framework is put in place to enable research to take place that could potentially be of benefit to many, many people in this country," his spokesman said.

Fierce moral clashes are also expected next week over a bid by pro-life MPs to use the Bill to lower the 24-week time limit on abortions to as few as 13 weeks.

As the first stage of the Parliamentary tussle got under way last night, on the broad principles of the Bill, Labour MP Geraldine Smith was among the most vocal critics.

Removing the need for a father for a child born through IVF treatment was "one of the most ill-conceived measures to be put before this House while I've been a member of it," she said.

The creation of hybrid embryos was "very revolting" and the creation of saviour siblings – born in order to help treat another child – "appalling".

Vowing to defy the Prime Minister and vote against the Bill's Second Reading, she said: "I fully respect the Government's position on this, but I think equally the Government should respect the position of members who, on an issue of conscience, will end up voting against the Government."

The Bill has been strongly criticised by some church leaders, most notably the leader of the Roman Catholic Church in Scotland, Cardinal Keith O'Brien, who denounced it as a "monstrous" attack on human rights.