The Vatican said on Monday that priests and other Roman Catholic Church ministers cannot bless same-sex unions and that such blessings are "not licit" if carried out.
The ruling was a response to practices in some countries, such as the United States and Germany, where parishes and ministers have begun blessing same-sex unions in lieu of marriage, and calls for bishops to defacto institutionalise these.
In response to formal questions from a number of dioceses on whether this practice was allowed, the Vatican's doctrinal office, the Congregation for the Doctrine of the Faith (CDF), issued the ruling: "Negative".
Pope Francis approved the response, the CDF said, adding that it was "not intended to be a form of unjust discrimination, but rather a reminder of the truth of the liturgical rite".
It said such blessings were not permissible even though they were "motivated by a sincere desire to welcome and accompany homosexual persons" and help them grow in the faith. The Roman Catholic Church has 1.3 billion members.
The CDF note said that since marriage between a man and a woman was a sacrament and blessings are related to the sacrament of marriage, they could not be extended to same-sex couples.
"For this reason, it is not licit to impart a blessing on relationships, or partnerships, even stable, that involve sexual activity outside of marriage (i.e., outside the indissoluble union of a man and a woman open in itself to the transmission of life), as is the case of the unions between persons of the same sex," it said.
At least two German bishops, including Cardinal Reinhard Marx of Munich, who is one of the pope's top advisors, have expressed support for some kind of "pastoral" blessing for same-sex unions.
But the CDF note approved by the pope ruled this out.
"The presence in such relationships of positive elements, which are in themselves to be valued and appreciated, cannot justify these relationships and render them legitimate objects of an ecclesial blessing, since the positive elements exist within the context of a union not ordered to the Creator's plan," the note said.
The Church teaches that gay tendencies are not sinful but gay acts are and that gay people must be treated with respect.
Last year, the Vatican had to move to clarify comment attributed to Pope Francis on civil union laws in a documentary, saying they were taken out of context and did not signal a change in Church doctrine on gay people or support for same-sex marriage.
The comments attributed to the pope in the film alarmed conservatives.
Reporters later discovered that two separate quotes in response to separate questions were spliced to appear as one, deleting the intervening context and questions.