The comedian featured in the poster is wearing a nurses' cap and a tunic which display a red cross.
In the letter the legal chief of the British Red Cross warned that the unauthorised use of the symbol could cost lives. The letter said: "Unauthorised use of the sign is an offence under the Geneva Conventions act 1957(section 6, 1a).
"The reason for the strict control is that the red cross emblem is an internationally agreed symbol of protection during armed conflicts. If the emblem is used for other purposes, its special significance will be diminished and potentially lives may be lost."
In response, the Glasgow Pavilion Theater management have changed his costume to feature a green cross.