The top Catholic official in San Francisco has prompted an outcry by conservative members of the Church after he gave Holy Communion to members of an activist group whose motto is 'Go Forth and Sin Some More'.

The Sunday Eucharist that was delivered on 7 October has stirred anger among the Church's followers after two men from the Sisters of Perpetual Indulgence group attended by cross-dressing as women. The men were so heavily hidden in their make-up and nun's habits that Archbishop George Niederauer delivered the Eucharist as normal to the mockers.

According to the San Francisco Chronicle, the Archbishop has written a letter of apology addressed to "Catholics of the Archdiocese of San Francisco and to Catholics at large".

He explained to outraged members that he did not realise his mistake until it was too late and the Mass had already been completed.

Archbishop Niederauer wrote: "At Communion time, toward the end of the line, two strangely dressed persons came to receive Communion. As I recall, one of them wore a large flowered hat or garland."

Although the group's reputation for causing trouble was known to the Archbishop, he had never himself encountered any of the group's members before.

"These two people have long made a practice of mocking the Catholic Church in general and religious women in particular," he said. "Someone who dresses in a mock religious habit to attend Mass does so to make a point (that) was intended as a provocative gesture."

The Sisters of Perpetual Indulgence is a group well-known to supporting the gay community, and have mocked the Catholic Church since the group was established in San Francisco in 1979.

Offensive names are given to each member, such as Sister Chastity Boner and Sister Constance Craving of the Holey Desire.

The San Francisco Chronicle explained that Sister Barbi Mitzvah, who serves as "Board Chairnun" and "Sexytary", said Tuesday that the group was "not offering a comment".

Speaking about the Catholic Church and conservatives, she said, "These people are always after us."

Rev Jim Bretzke, professor of moral theology at University of San Francisco, a Jesuit Catholic university has commented: "The general sacramental principle is that you don't deny the sacrament to someone who requests it. The second principle is that you cannot give communion to someone who has been excommunicated," as they are considered "manifest public sinners".

He added that while some may be offended "the sisters do not meet the criteria the church has for denying Communion. Over-accessorising and poor taste in makeup is not an excommunicable offence."

Rev Bretzke added, "Even if these people were bizarrely dressed, the archbishop was following clear pastoral and canonical principles in giving them Communion. The default is, you give Holy Communion to one who presents himself."

Matt Dorsey, who is a parishioner at Most Holy Redeemer Church gave a lighter opinion of events, saying: "I was there in jeans and T-shirt. I have to give the Sisters credit for taking far more time getting ready than I did."

He added that he was impressed by an e-mail sent to the general mailbox of Most Holy Redeemer after the Mass. One of the two 'Sisters' who attended the Mass wrote, "Just a quick note to recognise the wonderful mass yesterday at your Church. Your entire congregation was so welcoming and it was great to be able to participate. You are a wonderfully inclusive Church."