A conference of bishops has said that new legislation which will grant rights to unmarried straight and homosexual couples will weaken marriage.
The government of Uruguay has brought forward the new law, and it will be voted on by the lower house of parliament later this week.
The bill passed the Senate last September.
The new law was a manifesto commitment by the ruling leftist coalition of President Tabare Vasquez.
The Episcopal Conference of Uruguay (CEP) echoed bishops in Europe by attacking the validity of gay relationships.
“In no way can homosexual cohabitation be accepted because it does not meet the basic criteria defining marriage, it is therefore unacceptable to place it in suchlike equal level,” a statement from CEP said.
The main opposition party in Uruguay, Partido National, has said they will try to remove gay and lesbian couples from the new bill during this week’s debate in the Chamber of Deputies.
The bishops said: “We believe that the so-called advantage in favour of the unmarried and homosexual couples dwelling in a shared environment for a long time ought not to affect negatively the institution of family recognised by the Uruguayan Constitution and it does need to be taken care of and because of the incentive that it creates in society.”
Same-sex marriage will remain illegal in Uruguay, something LGBT rights groups say they will continue to fight.
However, the civil union bill is a major step in the right direction in a region where the Catholic Church dominates much of everyday life.
Because of the marriage ban judges have been unsure how to rule in a number of cases involving same-sex couples, particularly in areas of adoption, pensions and inheritance.
Senator Margarita Percovich, the author of the legislation, said the bill would give couples entering civil unions the same rights as marriage.
Under the legislation couples would have be together for at least five years and sign a registry.