The leader of Roman Catholics in England and Wales has urged the faithful to write to their MPs "as soon as possible" to oppose the Government's plans for gay marriage.
The Most Rev Vincent Nichols, Archbishop of Westminster, has called on Roman Catholics to express their views to their constituency MPs "clearly, calmly and forcefully" over the issue.
He said one of his concerns about the plans was how children would be taught about the "true nature of marriage" in schools if the law is changed to allow same-sex marriage.
"Of course there are many different circumstances to family life. Events reshape the family lives of many people," he wrote in a pastoral letter read out in parishes in the Diocese of Westminster over the weekend.
"We are right to express our admiration for those who work so hard to maintain family stability in difficulty and isolation.
"Support and loving care for them can make all the difference.
"But none of this takes away the importance of having a clear vision of marriage and family, based on human nature itself."
The Most Rev Nichols said he prayed that the nation would maintain the importance of marriage between a man and a woman as the "heart of family life" and resist the proposed redefining of marriage with all its "likely consequences", particularly in schools and how children are taught about the "true nature" of marriage.
"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," he said.
"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."
An estimated 5.2 million people identify themselves as Roman Catholics in England and Wales, according to research by the Cafod development agency, representing just under 10% of the population. But around 40% never or rarely go to church.
The call from the Most Rev Nichols comes after he used his Christmas Day message to describe the gay marriage plans as "Orwellian" and "shambolic".
The coalition Government is committed to legislating on gay marriage by the 2015 general election, and a Bill is expected to be tabled in January. Prime Minister David Cameron has promised his MPs a free vote on the issue.
A poll published last week showed voters support the plans to introduce gay marriage by a margin of two to one.
Some 62% of those questioned by pollsters ICM Research for The Guardian said that same-sex couples should be allowed to marry, against 31% who oppose the change and 7% who did not know.
The poll showed a majority of Conservative supporters in favour of the change, which has been championed by Mr Cameron in the teeth of significant opposition from Tory backbenchers.
Culture Secretary Maria Miller told the House of Commons earlier this month that she was putting in place a "quadruple lock" of measures to guarantee religious organisations would not have to marry same-sex couples against their wishes.
The legislation will also explicitly state that it will be illegal for the Church of England and the Church in Wales to marry same-sex couples.