The ad, created by Leo Burnett for Heinz, is running as part of a sponsorship campaign on ITV's 'Emmerdale'. It shows a guinea pig eating a lettuce leaf then drinking from a bottle of ketchup instead of a water bottle with the strapline: "You can't eat without it".
The RSPCA said that the ad could encourage children to replace essential guinea pig food with ketchup.
An RSPCA spokeswoman, said: "We are concerned about child copycat incidences arising from this ad. Hay, grass, cereal mix and water are an essential part of a guinea pig's diet and in no case should water be replaced with ketchup."
Heinz said it would not stop broadcasting the ad because it believed that viewers would see the humourous side as intended.
Despite its opposition, the RSPCA said it had no intention of trying to get the ad banned.
"No guinea pigs have been harmed during the making of this ad so we will not go directly to Ofcom, but all we can say at this stage is we are very unhappy about the ad," the spokeswoman added.
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