In a letter to be read to congregations at Mass this weekend, two senior Archbishops in the Catholic Church tell followers they have a ‘duty’ to ‘ensure’ gays should not be allowed to marry even in civil ceremonies.

The address as reported by the Daily Telegraph comes from the most senior Catholic in England and Wales, the Archbishop of Westminster, the Most Reverend Vincent Nichols, and the Archbishop of Southwark, the Most Reverend Peter Smith.

The letter will be read to 2,500 church Masses this weekend with a note encouraging faithful to sign the Coalition for Marriage, an effort to oppose marriage equality led by the heads of anti-gay religious groups including the Christian Institute.

Dismissing wholesale the arguments of “equality and discrimination”, the archbishops write: “But our present law does not discriminate unjustly when it requires both a man and a woman for marriage. It simply recognises 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 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.”

Describing marriage as an expression of “our fundamental humanity”, the letter says: “Neither the Church nor the State has the power to change this fundamental understanding of marriage itself.”

It continues: “Its status in law is the prudent fruit of experience, for the good of the spouses and the good of the family. In this way society esteems the married couple as the source and guardians of the next generation. As an institution marriage is at the foundation of our society.”

The address to church-going Catholics in England and Wales follows the head of the Catholic Church in Scotland’s comparisons of “grotesque” plans for gay marriage with the reintroduction of slavery.

The archbishops add: “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.”

The 2007 British Social Attitudes Survey estimated that 9% of the adult population of England, Wales and Scotland were Catholic and of those 14% attended weekly services.

A Daily Telegraph editorial today says the paper’s position is that gay marriage would “undermine the traditional family”. A poll on the story this morning of 33,000 readers puts support for gay marriage at 81%.