Mars bars get veggie status back
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.
Fiona Dawson, managing director of Mars UK, said the company had listened to customers and decided to reverse its decision.
"The consumer is our boss and we had lots of feedback from consumers who were unhappy about the change," Ms Dawson said.
"It became very clear, very quickly that we had made a mistake, for which I am sorry.
"There are three million vegetarians in the UK and not only did we disappoint them, but we upset a lot of the consumers."
The Vegetarian Society had called the move by Mars "incomprehensible" and "a backward step" at a time when consumers were increasingly concerned about "the provenance of their food".
Mars replied that, by admitting its products were no longer suitable for "extremely strict vegetarians", it was making a "principled decision".
Now it insists it will begin changing its recipes back immediately.
Dr Annette Pinner, chief executive of the Vegetarian Society, said she was pleased Mars had been "honest enough" to admit it had "made a mistake".
"A Masterfoods representative has made contact with us and we are very pleased that they now recognise the importance of integrity to all their customers, especially vegetarians," she added.
Rennet is extracted from calves' stomachs and was to have been used in the ice cream versions of all Masterfoods' bars too.