As it begins its passage through Parliament, campaigners have launched a joint campaign against the government's Human Fertilisation and Embryology Bill.
The legislation was due to receive its second reading in the House of Lords on Monday, but is set for a sustained attack over clauses that would give lesbian couples the right to have test-tube babies.
Peers, including the Archbishop of York Dr John Sentamu, and opposition MPs including former Conservative leader Iain Duncan Smith, argue that the plans are at odds with traditional family values.
They say the Bill removes the current legal right that every child should have a father.
Ahead of the debate in the Lords, Duncan Smith said: "Just as we are beginning to appreciate the vital role fathers play in the successful upbringing of children, Labour ministers are sending out the utterly wrong signal that fathers don't matter."
He is one of nearly 50 MPs from all parties who have tabled a Commons motion condemning the Bill.
Christian groups are also complaining that by allowing advent of "three-parent" children from two women and sperm from a donor through DNA, ministers are approving a "Frankenstein science" that could lead to human cloning.
Ministers are facing pressure to grant a free vote on the legislation, but the prime minister's official spokesman insisted that: "This is a government bill and will be treated as such."
