The carefully cultivated socially liberal image of Ben & Jerry's ice cream has suffered a knock with a decision by the Vermont-based manufacturer to stop calling its food "all natural" following pressure from a watchdog that questioned whether ingredients such as partially hydrogenated soya bean oil fitted the billing.

Founded by two college friends who set up a "scoop shop" at a dilapidated petrol station in 1978, Ben & Jerry's has been owned by the Anglo-Dutch conglomerate Unilever since 2000 and has become a popular premium ice cream on both sides of the Atlantic. Its founders are renowned for their activism on causes ranging from global warming to poverty reduction, liveable wages and free-range eggs.

Ben & Jerry's mission statement trumpets an aim to make "the finest quality, all-natural ice-cream and euphoric concoctions" and to promote business practices that "respect the earth and the environment". But the firm has come under fire from the Washington-based Centre for Science in the Public Interest (CSPI), which took issue with ingredients such as alkalised cocoa and corn syrup as well as partially hydrogenated soya bean oil.

The pressure group contended that the ingredients had either been chemically modified or did not exist in nature: "Calling products with unnatural ingredients 'natural' is a false and misleading use of the term."

In an abrupt about-turn, Ben & Jerry's has agreed to remove the term from its product descriptions. In a letter to the CSPI, the ice-cream company's chief executive, Jostein Solheim, said that although he believed "reasonable customers" would still consider Ben & Jerry's food to be natural, he did not want any further questions over the issue.

"We have decided to remove these claims and focus more strongly on our other core values," wrote Solheim, citing endeavours to use fair-trade suppliers, cage-free eggs and milk from family farms that do not use bovine growth hormones.

Ben & Jerry's is renowned for its wide range of quirky flavourings and punning names, ranging from Cherry Garcia (named after the Grateful Dead guitarist Jerry Garcia) to Chunky Monkey and Phish Food. Founders Ben Cohen and Jerry Greenfield have had an occasionally prickly relationship with Unilever – Cohen once described the day the business was bought by the multinational as "just about the worst day of my life".

Ben & Jerry's is left facing unfortunate comparisons with its US rival Häagen-Dazs, which last year made a great fanfare about a new range called "five", which contained just five ingredients: cream, skimmed milk, sugar, egg yolks and a natural flavouring such as vanilla or strawberry.