LONDON — The Roman Catholic Church stepped up its campaign against civil gay marriage, with a letter from two senior archbishops being read out at services in 2,500 churches on Sunday.

The letter from Archbishop Vincent Nichols, the leader of the Catholic Church in England and Wales, and Archbishop Peter Smith, the Archbishop of Southwark, said it was their "duty" to defend the institution of marriage.

"Changing the legal definition of marriage would be a profoundly radical step. Its consequences should be taken seriously now," Nichols and Smith said in the letter, which was being read out at parish churches in England and Wales.

"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."

Prime Minister David Cameron's government will later this month launch a formal consultation on plans to introduce same-sex civil marriages before the next general election in 2015.

One week ago Cardinal Keith O'Brien, the leader of the Catholic Church in Scotland, said the plans were "madness".

But a poll in the right-leaning Sunday Telegraph newspaper showed public support for the proposals, with 45 percent saying they supported the move to legalise gay marriage, 36 percent opposing it.

Civil partnerships for same-sex couples were introduced in Britain in December 2005, giving them similar rights to married heterosexual couples. But the partnerships cannot legally be referred to as marriages.

New laws allowing same-sex civil partnership ceremonies to be conducted in places of worship in England and Wales came into force in December, though no religious group is obliged to host them.