Falmouth MP Sarah Newton is taking a stand against Asda over its plans to open a pharmacy in store.
The company had applied to Cornwall Council for permission to lift a condition stopping it having a counter in the Penryn store.
However that was turned down over concerns it would draw people away from the town centre, affecting local businesses, with Hendra Chemist stating it would be forced to close if permission was granted.
That refusal is currently the subject of an appeal with a decision due in weeks. In the meantime the store has controversially opened a temporary pharmacy in a portable building in the car park.
Ms Newton, who has started a petition against the application, said: "I am urging Cornwall Council to do everything it can to stand up for Penryn by robustly defending its decision to refuse permission."
She will be delivering the petition to households in Penryn on Saturday morning and has invited Mayor Gill Grant to accompany her to raise awareness.