The Tories and Lib Dems have given their MPs a free vote on all parts of the Human Fertilisation and Embryology Bill.

But Gordon Brown allowed Labour MPs and ministers a free vote on only four key issues for the Bill’s committee-stage reading in the Commons. His decision followed a fierce campaign led by the Catholic Church.

When the Bill returns for its third and final reading later in the summer, prominent Labour Catholics like Ruth Kelly will have to fall into line or face the sack. The Transport Secretary voted in favour of the outright hybrids ban and the compromise.

Apart from Ms Kelly, those who voted in favour of the ban and the compromise included Defence Secretary Des Browne, Welsh Secretary Paul Murphy and Europe Minister Jim Murphy.

Tory leader David Cameron voted against the full ban but abstained on the compromise, despite it being put forward by his frontbench.

Scientists will now be allowed to insert human DNA into animal eggs.

Smaller than a pinhead, the hybrid embryos will be allowed to grow for up to 14 days before being destroyed.

Scientists believe that stem cells harvested from these embryos could provide the key to the treatment of conditions such as Parkinson’s disease, cystic fibrosis or muscular dystrophy.

Research is currently being held up by the shortage of human eggs to create stem cells.

MPs were also due to vote on so-called “saviour siblings” – babies genetically engineered to provide material for ill family members.

Tory MP Edward Leigh, who gave the Frankenstein warning, said the Bill would allow for the creation of true hybrids – in which an animal egg is fertilised by a human sperm.

Opening the debate with an amendment prohibiting outright the creation of hybrid “admixed” embryos, Mr Leigh claimed there was “no evidence yet to substantiate” the claims that they could lead to cures.

But Labour MP and former biology lecturer Ian Gibson said: “Some people have done that, actually.

“It is kind of secret in some ways because there is commercial interest, but some drugs have been developed using embryos and the effects of them.”

Today debate will centre on making it easier for lesbians to have IVF babies, as well as the abortion limit.

Mr Brown will vote for the removal of a requirement for IVF clinics to consider a child’s “need for a father”, while Mr Cameron believes it should stay.

The Tory leader has also indicated that he would like to see a reduction in the current 24-week limit for abortion, while Mr Brown wants it to remain.