(Decanter) - Vatican officials have told Catholic priests that wine used for Holy Communion must be 'natural' and should not be sour, and that they should avoid gluten-free bread.

New rules on Holy Communion wine and bread issued by the Vatican come due to concerns about the widespread availability of products that might be unfit for the job.

‘Until recently it was certain religious communities who took care of baking the bread and making the wine for the celebration of the Eucharist,’ said the Vatican.

‘Today, however, these materials are also sold in supermarkets and other stores and even over the internet.’

The new guidance has been sent in a letter to Bishops by the Vatican’s Congregation for Divine Worship and the Discipline of the Sacraments, at the behest of Pope Francis. Vatican Radio published the letter in full.