Fifty Greenpeace members have staged a protest at Sainsbury's, claiming cows which produce its own brand milk are fed genetically modified maize.
Protesters say Sainsbury's milk comes from cows fed GM maize
Dressed as cows, they chained themselves to the dairy aisle and entrance and scaled the roof of the store in Greenwich, south east London.
Campaigner Ben Stewart said Sainsbury's claim to sell environmentally-friendly food was "hypocritical".
Sainsbury's said it was the first major supermarket to ban GM ingredients.
A company spokesman said: "This step was taken in response to overwhelming public customer demand, and we have every confidence in the integrity of our milk."
But Greenpeace alleges UK cows which produce milk for the supermarket are fed on genetically modified maize imported from the US.
"Sainsbury's can help stop GM in its tracks by turning its back on this GM milk," said campaigner Emma Gibson.