The most senior Catholic in England and Wales, Cardinal Vincent Nichols and his Scottish counterpart, Archbishop Philip Tartaglia also called upon healthcare professionals and others involved in terminations to seek "the forgiveness of God".
In a joint statement, they wrote: "In the case of abortion, decisions and choices need to acknowledge the duty to cherish human life and to foster its flourishing beyond the circumstances of any one person, however challenging these may be."
Marking the 50th anniversary of the Abortion Act on Thursday, the two leaders also called for a change of attitudes, in favour of greater protection for human life.
More than eight million pregnancies have been aborted since the legislation, previously known as Lord David Steel's private member's bill, received Royal Assent.
Urging a period of prayer and fasting, the 2,000 word document signed by the Catholic leader also expressed concern that they are not enough safeguards to protect unborn children affected by disabilities.
Cardinal Vincent Nichols and Archbishop Philip Tartaglia said there was an urgent need for parents and teachers to promote the sanctity of human life.