A letter opposing David Cameron’s plans to legalise same-sex marriage is to be read out in 2,500 churches across England and Wales this weekend, according to a report in today’s Daily Telegraph.

The letter by the Archbishop of Westminster, Dr Vincent Nichols (pictured), and the Archbishop of Southwark, Dr Peter Smith, said the Cameron proposal threatens the essential meaning of marriage.

Redefining marriage to include homosexuals would be a “profoundly radical step” stripping it of its “distinctive nature”, the letter will say.

A  national consultation on the proposed change to the law on marriage begins this week.

The letter argues that marriage is a “natural institution” which “provides the best context for the flourishing of their relationship and for bringing up their children”. 

It says: “Society recognises marriage as an important institution for these same reasons: to enhance stability in society and to respect and support parents in the crucial task of having children and bringing them up as well as possible”.

It also argues that allowing same-sex couples to marry would reduce marriage to a vague commitment between two people. The archbishops argue that marriage between a man and a woman is “at the foundation of our society”.

“Neither the Church nor the State has the power to change this fundamental understanding of marriage itself,” they write. “Nor is this simply a matter of public opinion.”

Crucially, they argue against changing the meaning of civil, as well as religious, marriage. The Government had hoped to neutralise opposition from a coalition led by the former Archbishop of Canterbury Lord Carey, by offering reassurances that churches would not be forced to marry same-sex couples.

The archbishops’ letter 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 their parishioners to sign a petition set up by Lord Carey’s Coalition for Marriage, opposing the redefinition of marriage.

More than a million people attend Mass each week, out of an estimated five million Catholics in England and Wales. In 2007, a letter issued by the then Archbishop of Westminster, Cardinal Cormac Murphy-O’Connor, helped secure the future of faith schools as the Labour government came under pressure from unions to end state funding.

Churchgoers will be encouraged to take part in the Government consultation when it begins and lobby their MPs directly.

“The reasons given by our Government for wanting to change the definition of marriage are those 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.”

Changing the law would “gradually and inevitably” change society’s understanding of the purpose of marriage, they say.

“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.”

Meanwhile, Scotland’s most senior Catholic cleric has described Government’s proposals as a “grotesque subversion of a universally accepted human right”.

Cardinal Keith O’Brien accused David Cameron of trying to “redefine reality” at the “behest of a small minority of activists”.

In a strongly worded article for The Sunday Telegraph Cardinal O’Brien said: “As an institution, marriage long predates the existence of any state or government. It was not created by governments and should not be changed by them.

“Instead, recognising the innumerable benefits which marriage brings to society, they should act to protect and uphold marriage, not attack or dismantle it.”

The Coalition for Marriage, set up to oppose the Government’s plan to redefine marriage and headed by Lord Carey, has already attracted over 100,000 signatures.