Anti-GM protesters dressed as cows staged a demonstration outside a Sainsbury's in Bristol on Saturday.
Greenpeace staged a similar protest in London
About a dozen members of Greenpeace protested inside and outside the Castle Court store using inflatable milk bottles. The police were called.
In a statement, the environmental pressure group said: "Sainsbury's own-brand milk comes from cows fed on American GM animal feed."
The store has said it is the first major supermarket to ban GM ingredients
"This step was taken in response to overwhelming public customer demand, and we have every confidence in the integrity of our milk," a Sainsbury's spokesman said when Greenpeace staged a similar protest in London recently.
Greenpeace campaigner Ben Ayliffe said: "Sainsbury's can help stop GM in its tracks by turning its back on this GM milk."