The world's largest artificial sweetener producer has issued legal proceedings against Asda for defamation after the supermarket labelled aspartame a 'nasty'.

Ajinomoto Sweeteners, which had a £5.5bn turnover in the year to March 2007, said it had tried unsuccessfully to negotiate an agreement with Asda over claims by the supermarket related to its Good for You range. It said: "Ajinomoto has taken this step to defend the reputation not only of the ingredient, but also of the many products that are sweetened with aspartame."

As part of its campaign to tell customers there would be "no more nasties" in its food, Asda listed on its Good For You range the ingredients it had removed, including: aspartame, artificial colours, preservatives and hydrogenated fats.

"We have led the way in the past year by removing all artificial colours and preservatives from our own-label food," said an Asda spokesman.

"We have removed ingredients our customers tell us they don't want. That includes aspartame ."

But Ajinomoto said aspartame had been unfairly vilified. "Aspartame helps people control their weight. At a time when we are seeking ways to combat obesity, it is unconscionable that Asda should vilify a safe and beneficial ingredient."

Ajinomoto said it had not put a figure on the damages it was seeking.