If a week is a long time in politics, then in the food industry it must feel like an eternity. At least it will for Masterfoods, which is coping with the fallout from its decision to add animal-based rennet to its bestselling chocolate bars. After The Grocer revealed the blunder, the company felt the full force of the vegetarian backlash. By the following weekend, a swift u-turn was the only option to survive the PR crisis and a very apologetic Masterfoods announced it was reverting to the old recipe. But how damaging will the whole affair be?

In terms of PR, the company set a precedent in the speed of its response. Just days after the news broke, it took out a full page ad in many national papers, apologising and promising to switch back.

"It became very clear, very quickly, we had made a mistake, for which I am sorry," said the statement by Fiona Dawson, MD of the Mars snack food business in Britain. But in the short term, its public image has taken a battering. Hundreds of blogs, online petitions and even website pages were created to persuade the company to change its mind. More than 6,000 members of the public and 40 MPs complained to the company.

The brand's Buzz rating, measured by YouGov's Brand Index, fell dramatically. According to the Buzz rating - a measure of whether people have heard positive or negative things about a brand - the company fell from a score of 9.0 to -0.2 in a few days. The incident highlights the danger of meddling with a product people love - particularly one in which the use of meat derivatives seems both shocking and unnecessary.

"I think the reason this has been so problematic for Masterfoods' reputation is because many consumers see the decision as financially motivated," says one branding expert. "People do not like the thought a big business can make cavalier decisions without informing the public."

Liz O'Neill, communications director at the Vegetarian Society, adds: "This has been a very extreme example of how consumer power, in association with independent groups like us, can force change. The company underestimated the integrity of vegetarians and how important the provenance of food is to the consumer as a whole."

The question now is how quickly Masterfoods will be able to follow its words with action. In the statement, Dawson said: "I personally commit to make sure the products we changed will be suitable for vegetarians again in the near future."

But though vegetarian-friendly Mars bars are expected to return to the shelves in the next two to three weeks, non-vegetarian product will remain on shelf. Meanwhile, Twix, Bounty and Celebrations, which were also reformulated, continue to be unsuitable for vegetarians. With three million vegetarians in Britain, that is a fair number of people suddenly excluded from buying a product.

Masterfoods refuses to say how much it thinks the whole exercise will cost, either in terms of lost sales or the forced switch back to a vegetarian rennet. It has also hit back at claims that the lower-cost animal rennet was introduced simply as a cost-cutting measure.

Most believe the company will bounce back. "In the long run I don't think it will have an impact on the sales or reputation, because they are such a well-known big company," says Sophie Goodale at top UK PR consultancy Max Clifford Associates.

That, of course, is why they got into trouble in the first place.