Leading chocolate brand Mars would stay vegetarian after all. Its manufacturer, Masterfoods, had raised a storm when it announced that it had started using rennet - an enzyme taken from the stomachs of newborn calves - to make whey for its products. It had previously been relying on a readily available - but more expensive - vegetarian alternative to rennet.

But it provoked a furious reaction, forcing a quick U turn. "I'm not a vegetarian, but prefer to avoid products with animal derivatives where an alternative could easily have been used. That will now include all Mars products.

Sad, because I much prefer their chocolate," reacted a customer.

Vegetarian Society spokesman Liz O'Neill said: "People will not like the idea that you cannot make a Mars bar without killing a calf.

"We have had no good, clear answer from Masterfoods why they have to use animal products when all these items were previously produced using vegetarian alternatives. "But we can no longer describe any of them as suitable for vegetarians."

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."

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.