A furious protest by vegetarians has forced the company that makes Mars Bars to stop using rennet, an animal enzyme taken from the stomach lining of new-born calves, in some of its chocolate bars.

Masterfoods announced last week that it had started to use the substance instead of whey, a more costly alternative derived from cheesemaking, for some of its chocolate and ice cream. The change affected the Mars, Snickers, Maltesers and Galaxy brands.

The Vegetarian Society protested on behalf of the three million vegetarians in the UK and asked people to “express their concern” to the company, which received 6,000 complains within a week.

Forty MPs also signed a petition “to voice their disquiet”, said Mars UK.

Fiona Dawson, managing director of the Mars snack business in the UK, said today: “The consumer is our boss and we had lots of feedback from consumers who were unhappy about the change. 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 had said that admitting it could not guarantee all products as suitable for vegetarians was a “principled decision”. It said today that it would 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.”

The society also invited the company to seek vegetarian approval through its accreditation scheme.

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