Mr Cameron said equality must come first

Catholic adoption agencies have said they will close rather than place children with gay couples, saying it goes against their beliefs.

Shadow home secretary David Davis says he will "almost certainly" vote against the anti-discrimination regulations.

But Mr Cameron said there should be "clear rules against discrimination".

I don't think personally that it is right to give them a block exemption from the law David Cameron

Mr Cameron told BBC Radio 4's Today programme: "On the issue of the Catholic adoption agencies, I don't think personally that it is right to give them a block exemption from the law, because otherwise we will have other people wanting block exemptions from the law."

But there was a need for a "decent compromise", such as giving the agencies up to four years to comply with the law, he said.

Free vote

Conservative MPs will be given a free vote if and when the new Sexual Orientation Regulations go before Parliament next month.

Mr Cameron's home affairs spokesman, Mr Davis, said on Sunday that he would "almost definitely" vote against forcing faith-based adoption agencies to offer babies to gay couples.

He said rights must be balanced against children's right to the best available adoption service.

The Equality Act, due to come into force in April, would forbid schools, businesses and other agencies from refusing services to people on the grounds of sexual orientation.

But Cardinal Cormac Murphy-O'Connor, head of Catholics in England and Wales, has asked that his adoption agencies be allowed to opt-out of rules. He has been supported by the Church of England .

The row is said to have split the Cabinet. Prime Minister Tony Blair has said the government will put forward its preferred option this week.