Mars has abandoned its plan to use animal products in its chocolate after the change sparked an outcry from vegetarians.
The company announced last week it was changing the whey used in some chocolate and ice cream from a vegetarian source to one with traces of rennet, an animal enzyme.
The firm has apologised to upset vegetarians who had launched a campaign against the move.
The Vegetarian Society protested on behalf of three million vegetarians in the UK and asked people to "express their concern" to Masterfoods, which produces the Mars, Snickers, Maltesers and Galaxy brands affected by the change.
Within days of the decision being announced, more than 6,000 people had called and emailed the Mars UK switchboard, which usually receives 500 comments per week.
Fiona Dawson, managing director of the Mars snack business in the UK, said: "The consumer is our boss and we had lots of feedback from consumers who were unhappy about the change."
She added: "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.
"We have listened to their views and have decided to reverse our decision."
The company has announced that it will begin changing its recipe back immediately for the best-selling four brands and at a later date for Minstrels.
Ms Dawson added: "We made the original decision in order to broaden our supply base to ensure the availability of our supply, but we underestimated the impact this would have.
"I personally commit to make sure that the products we changed will be suitable for vegetarians again in the near future."
Dr Annette Pinner, chief executive of the Vegetarian Society, said: "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.
"We cannot endorse any planned actions by the company until we receive detailed assurances about the ingredients and processes involved in production but we are delighted that Mars UK has been honest enough to mark the beginning of National Vegetarian Week by admitting that it made a mistake."
According to this only the 4 main ones will be Veggie; doesn't mention Milky Way, Starburst, Bounty etc. so we'll have to wait and see... Also most of the ones in the shops at the mo contain rennet still. There will be a lengthy period when they're still not ok for Veggies!