The leader of the Roman Catholic church in England and Wales has urged parishioners to write to their MPs “as soon as possible” urging them to block the Government’s same-sex marriage plans.
The Most Rev Vincent Nichols, the Archbishop of Westminster, said they should write “clearly, calmly and forcefully” outlining their concerns.
Among his fears is that children will no longer be taught about what he called “the true nature of marriage” in schools, if legislation is passed allowing marriage between homosexuals.
With the bill on same-sex marriage due to be tabled in mid-January, the clock is now ticking.
At least 130 Conservative MPs could vote against the Government on the issue, but many more would be needed to stop it becoming law.
In a pastoral letter to mark the Feast of the Holy Family, Archbishop Nichols called on Catholics to act now to change politicians’ minds.
It is the first time he has written directly asking parishioners to lobby MPs, and marks a new phase in the church’s campaign against the potential law change.
About five million people in England and Wales are Catholic, of which a million regularly attend church.
In the letter, read out in some churches, Archbishop Nichols said he wanted politicians to “resist the proposed redefining of marriage with all its likely consequences particularly in schools and in how children are taught about the true nature of marriage”.
He continued: “At this time, we look to our Members of Parliament to defend, not change, the bond of man and woman in marriage as the essential element of the vision of the family.
“I urge everyone who cares about upholding the meaning of marriage in civil law to make their views known to their Members of Parliament, clearly, calmly and forcefully. Please do so as soon as possible.”
Christmas, he wrote, was “a time in which to speak up for marriage, between a husband and a wife, as the heart of the family”.
It was important, he reiterated, to have “a clear vision of marriage and family, based on human nature itself”.
“The vocation of marriage”, he argued, was “rooted in a natural bond, blessed by God”.
Archbishop Nichols’ letter comes days after he attacked the Coalition’s plans to legalise gay marriage as “shambolic”, arguing ministers had no mandate from the public as there was no mention in either party’s manifesto.
He said: “From a democratic point of view, it’s a shambles. George Orwell would be proud of the manoeuvre. I think the process is shambolic.”
The Catholic church has been strongly opposed to extending marriage to gay couples since the proposal was unveiled in March.
At the time, Archbishop Nichols and the Archbishop of Southwark, the Most Reverend Peter Smith, wrote to congregations saying that redefining the “natural institution” of marriage would be a “profoundly radical” step.
They wrote: “Neither the Church nor the State has the power to change this fundamental understanding of marriage itself. Nor is this simply a matter of public opinion.”
Catholic schools have also been asked to promote a petition organised by the Coalition For Marriage opposing the plans.
Civil partnerships, introduced seven years ago, already enable gay couples to celebrate their unions in civil setting like town halls.
Under Mr Cameron’s plans, gay couples would instead be able to get married – either in a civil ceremony or, if agreed by religious authorities, in a religious setting.
Some religious organisations such as the Quakers and the Unitarians want to be able to offer marriages to gay couples. The Catholic church is strongly against, while the Church of England is divided on the subject.
Although Mr Cameron has repeatedly emphasised that churches will not be forced to marry gay couples, many Christians fear that equality legislation will eventually be used to force their hand.