Mars bars get veggie status back
The move affected Masterfoods' chocolate and ice cream bars
The firm had said it would change the whey used in some of its products from a vegetarian source to one with traces of the animal enzyme, rennet.
The Vegetarian Society organised a campaign against the move, asking members to voice their concerns to parent company Masterfoods.
Mars said it became "very clear, very quickly" that it had made a mistake.
In just one week, more than 6,000 people bombarded the company, which produces the Mars, Snickers, Maltesers and Galaxy brands, with phone and e-mail complaints.
Forty MPs also signed a petition to voice their opposition.