Several Spanish mayors have said they will refuse to marry same sex couples despite a bill passed by deputies which would allow gay couples to wed.
The bill was welcomed by Spanish gay rights campaigners
One of the mayors, Javier Leon de la Riva, said that while he supported equal rights for gay couples, their unions cannot be called a marriage.
Last week, parliament's lower house moved to allow gay couples to marry and adopt children.
The Vatican condemned the bill, which is likely to become law within months.
The measure now goes to the Spanish Senate.
If passed, it would make Spain - a predominantly Roman Catholic nation - the first European country to allow homosexuals to marry and to adopt children.
Belgium and the Netherlands are the only other members of the European Union to allow same-sex marriages.
"Even if the law allows me to marry homosexuals, I will not exercise this authority," the mayor of the northern city of Valladolid, Javier Leon de la Riva, was quoted by Spanish newspapers as saying.
"I do not have a problem with these couples having the same rights as the rest of the citizens. But what does not seem right is that their union should be called a marriage," he added.
Meanwhile, the mayor of the Catalan town of Pontons, Lluis Fernando Caldentey, described the gay marriage bill as immoral.
At least another two mayors also voiced strong criticism the bill, reports in local media said.
Spain's Deputy Prime Minister Maria Teresa Fernandez de la Vega earlier warned that everyone country must comply with the bill.
But the legislation directly contradicts the Vatican's view on marriage and has brought the Socialist administration into direct conflict with Spain's Roman Catholic Church.
Spain's Roman Catholic bishops have launched a campaign to oppose the bill, calling on Catholics to protest against the changes by taking to the streets.
Last week, the head of the Vatican's Pontifical Council on the Family, Cardinal Alfonso Lopez Trujillo, denounced the legislation as "profoundly iniquitous".