Catholics in England and Wales will be urged to oppose gay marriage during masses in 2,500 churches on Sunday (11 March).
In a letter written by Archbishop of Westminster the Most Reverend Vincent Nichols, the leader of the Catholic Church in England and Wales, congregations are called on to protect the true meaning of marriage by taking part in a national consultation on the British government's plans for equal marriage rights and to lobby their MPs.
Co-signed by the Archbishop of Southwark, the Most Reverend Peter Smith, the letter will be read to the country's more than a million Catholics who attend services every week.
It says that extending marriage to same-sex couples would reduce wedlock to a vague commitment between two people and undermine it as a 'natural institution'.
The letter also warns against changing not only the definition of religious marriage but that of civil marriage as well.
The address, which has been seen by The Telegraph, has been sent to bishops across England and Wales and is being circulated to parishes this week.
It is accompanied by a cover note asking priests to encourage churchgoers to sign a petition set up by Lord Carey’s Coalition for Marriage, opposing the redefinition of marriage.
'The reasons given by our government for wanting to change the definition of marriage are those of equality and discrimination,' the letter reads.
'But our present law does not discriminate unjustly when it requires both a man and a woman for marriage. It simply recognizes and protects the distinctive nature of marriage.
'Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously now.'
The archbishops argue changing the definition of marriage would alter the purpose of marriage, which they claim is 'for the procreation and education of children' between a man and a woman.
The letter continues: 'The roots of the institution of marriage lie in our nature.
'This pattern is, of course, affirmed by many other religious traditions — understood as a lifelong commitment between a man and a woman, and for the creation and upbringing of children, marriage is an expression of our fundamental humanity.'
Nichols' statement is moderate in comparison to the leader of Scottish Catholics, Cardinal Keith O'Brien, who has sparked outrage after likening gay marriage to slavery, claiming it is a violation of human rights.
The archbishops join a list of high ranking religious officials who have voiced their concerns over the implications of legalising gay marriage in Britain.
The government's consultation on the proposals, which would give married same-sex couples equal legal rights, is about to be launched and high profile MPs, including the leaders of all three main political parties, have pledged their support.