Pope Francis has given his approval for priests to bless couples in "irregular" situations, including same-sex unions.
The doctrinal declaration, 'Fiducia supplicans', was issued by the Dicastery for the Doctrine of the Faith on Monday and has the Pope's approval.
It permits blessings for same-sex couples so long as they are conducted without any ritualisation and do not give the impression of marriage.
The Vatican said that the declaration does not signify approval of same-sex unions and that the Catholic Church's doctrine on marriage as between a man and a woman has not changed.
The declaration gives "the possibility of blessing couples in irregular situations and same-sex couples without officially validating their status or changing in any way the Church's perennial teaching on marriage", the Vatican said.
The form of the blessings "should not be fixed ritually by ecclesial authorities to avoid producing confusion with the blessing proper to the sacrament of marriage".
"In such cases, a blessing may be imparted that not only has an ascending value but also involves the invocation of a blessing that descends from God upon those who - recognizing themselves to be destitute and in need of his help - do not claim a legitimation of their own status, but who beg that all that is true, good, and humanly valid in their lives and their relationships be enriched, healed, and elevated by the presence of the Holy Spirit," the declaration reads.
"These forms of blessing express a supplication that God may grant those aids that come from the impulses of his Spirit - what classical theology calls 'actual grace' - so that human relationships may mature and grow in fidelity to the Gospel, that they may be freed from their imperfections and frailties, and that they may express themselves in the ever-increasing dimension of the divine love."
It adds later, "This is a blessing that, although not included in any liturgical rite, unites intercessory prayer with the invocation of God's help by those who humbly turn to him. God never turns away anyone who approaches him!"
The blessings can be offered to individuals who "although in a union that cannot be compared in any way to a marriage, desire to entrust themselves to the Lord and his mercy, to invoke his help, and to be guided to a greater understanding of his plan of love and of truth".
They are not to become the norm but should be a "a practical discernment in particular circumstances".
The announcement comes a day after prayers of blessing for same-sex couples were permitted in Church of England services for the first time.
The Catholic Church's declaration has been criticised on social media.
John Stevens, National Director of the Fellowship of Independent Evangelical Churches (FIEC), said, "As with the blessings introduced yesterday in the Church of England, it is a contradiction in terms.
"The only way that those in same-sex relationships can 'mature and grow in fidelity to the Gospel' is by repenting of sin and following the Bible commands that sex is only appropriate within marriage between a man and a woman."
Joshua Abbotoy, executive director of American Reformer, said it was "hard not to read his actions as an ever more tortured web of justifications and equivocations that all tend in the direction of making the Church inoffensive to progressive sensibilities - walking right up to the line of being affirming in function without formally renouncing the Magisterium".
"Don't get me wrong - Protestants fight this very same battle with our own leaders. But for us, the stakes are lower. The effects of bad leadership are much more easily mitigated," he said on X, formerly Twitter.