Legalization of same-sex marriage could obscure the "real meaning" of marriage for generations to come, the bishops of England and Wales are telling Catholics.

In a letter read aloud in about 2,500 parishes the weekend of March 10-11, the bishops told the faithful that an imminent government proposal to change the law represents "a profoundly radical step.

Its consequences should be taken seriously now," said the letter, co-signed by Archbishops Vincent Nichols of Westminster and Peter Smith of Southwark, president and vice president respectively of the Bishops' Conference of England and Wales.

"The law helps to shape and form social and cultural values," the letter says.

"A change in the law would gradually and inevitably transform society's understanding of the purpose of marriage.

"It would reduce it just to the commitment of the two people involved," the letter says.

"There would be no recognition of the complementarity of male and female or that marriage is intended for the procreation and education of children."