Vegetarians in Britain are set to boycott products such as Twix, Snickers, Mars Bars and Maltesers following the manufacturer's decision to begin using animal-derived whey. The company began using whey, which contains rennet - an ingredient obtained from animal's stomachs - at the beginning of May. According to the UK's Vegetarian Society, there are around three million vegetarians in the country - making them a group with considerable consumer clout. The Society is now urging consumers to contact Masterfoods directly with their concerns. In a statement, the organisation said, "at a time when more and more consumers are concerned about the provenance of their food, Masterfoods' decision to use non-vegetarian whey is a backward step." "Mars products are very popular with young people and many will be shocked to discover that their manufacture now relies on the extraction of rennet from the stomach lining of young calves," he added. Masterfoods has adopted the use of whey in most of its core brands due to changes in the sourcing of its ingredients. The company says products with an expiry date before October 1st are still vegetarian-friendly, and they are offering refunds to consumers who bought the new products in ignorance of the change. Rennet is a complex of enzymes produced by mammals that allows them to digest milk. Vegetarian rennet is generally produced from moulds but is more complex and expensive to produce on a large scale.