In both the United States and the European Union, the description “gluten-free” can be legally applied to foods made with wheat starch from which almost, but not absolutely, all gluten has been removed — the upper limit is 20 parts per million. The Catholic Church will allow bread of this kind to be used for communion.

But it will not allow truly gluten-free altar breads made with rice, potato, tapioca or other flours in place of wheat. (The Church of England has taken a similar position, while some other Christian denominations consider such breads acceptable.)

“The confusion can be great when these ‘breads’ are advertised as gluten-free alongside what are described as gluten-free but are in fact low-gluten altar breads,” according to the Catholic Church in England and Wales. “The confusion can also be the cause of great upset both to those Catholics who are allergic to gluten and to those who minister to them.”

The new instructions — given in a letter to bishops from the Vatican’s Congregation for Divine Worship and the Discipline of the Sacraments — said that the confusion had worsened because communion breads had become widely available, with varying standards of marketing and labeling.

“Until recently, it was certain religious communities who took care of baking the bread and making the wine for the celebration of the eucharist,” the congregation said. “Today, however, these materials are also sold in supermarkets and other stores, and even over the internet.”