Catholics will be urged to protect the "true meaning" of matrimony as the Catholic Church steps up its campaign against Government plans for gay marriage.
A letter from two senior archbishops will be read in 2,500 parish churches during Mass, arguing that the change would reduce the significance of marriage.
Archbishop Vincent Nichols, the leader of the Catholic Church in England and Wales, and Archbishop Peter Smith, the Archbishop of Southwark, will tell Catholics they have a "duty to do all we can to ensure that the true meaning of marriage is not lost for future generations".
Changing the legal definition of marriage would be a "profoundly radical step", they warn, and would strip the union of its "distinctive nature."
"Changing the legal definition of marriage would be a profoundly radical step," the Archbishops say. "Its consequences should be taken seriously now. The law helps to shape and form social and cultural values. A change in the law would gradually and inevitably transform society's understanding of the purpose of marriage.
"It would reduce it just to the commitment of the two people involved. There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children. We have a duty to married people today, and to those who come after us, to do all we can to ensure that the true meaning of marriage is not lost for future generations."
The letter argues that the roots of marriage lie in human nature and the pattern of "complementarity and fertility" in the union are affirmed by many other religious traditions and it insists that same-sex couples are not unfairly discriminated against under the current law.
Prime Minister David Cameron's official spokesman has said in response: "The Government has made clear its commitment to equality. We believe people should have the option of civil marriage, irrespective of sexual orientation."
Ben Summerskill, chief executive of gay rights organisation Stonewall, said: "At a time when 50,000 families in Britain are homeless and a billion people across the world live on less than a dollar a day, it's extraordinary that Archbishops are worrying about the family arrangements of a few thousands gay people.
"We assume that Roman Catholic congregations will take as much notice of the instruction to marginalise gay people's relationships as they do of the regular instruction they receive not to use birth control."