Archbishop Vincent Nichols has outlined his opposition in a pastoral letter to be read out during mass this Sunday.
It warns that the proposal "threatens the true meaning of a sacred union" and signals a "profoundly radical step" that will strip marriage of its "distinctive nature".
The letter is co-signed by the Roman Catholic Archbishop of Southwark, the Most Rev Peter Smith.
It argues that marriage as a union between a man and a woman should remain the "foundation of our society".
The head of the Roman Catholic in Scotland, Cardinal Keith O'Brien, was more scathing in his attack on the Government's plans at the weekend.
He said it was clear that the proposal was "not about rights but rather is an attempt to redefine marriage at the behest of a small minority of activists".
"Redefining marriage will have huge implications for what is taught in our schools, and for wider society," he said.
"It will redefine society since the institution of marriage is one of the fundamental building blocks of society. The repercussions of enacting same-sex marriage into law will be immense."
The Cardinal urged people to sign a petition launched last month to oppose any attempt to redefine marriage.
The petition, launched by the Coalition for Marriage, has already amassed more than 105,000 signatures.