Environment group Friends of the Earth is calling on councillors to turn Brighton and Hove into a GM-free city.
Activists will lobby councillors tomorrow as they arrive at Hove Town Hall, where the full council will debate two motions urging a ban.
The council's own Sustainability Commission and the Green Party have both tabled motions calling for a ban on growing or using genetically modified crops and food.
Friends of the Earth campaigner Liz Brookes said the Government would decide soon whether to permit commercial growing of GM crops in Britain.
She said: "If that happens, people will no longer have the choice of buying organic products.
"We want Brighton and Hove City Council to make a stand against GM crops and food by declaring the city a GM-free zone."
Parts of the South West and the whole of Wales have already been declared GM-free, using EU rules which green groups claim allow local authorities to ban the controversial technology.
People who attended a consultation meeting in Brighton in the summer, organised as part of the Government's GM Nation programme, voted overwhelmingly for a ban.
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