The Councillor for Hanover and Elm Grove, Matt Follett has resigned due to moving outside of the city with his family.
A by election will be called within 35 days time excluding weekends and bank holidays.
Council leader Jason Kitcat said: "I wish Matt and his family all the very best for the future.
"I want to thank him for his hard work serving the residents of Hanover and Elm Grove ward over the past two years"
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Network Rail is to reopen the level crossing in Barns Green in West Sussex on Thursday this week, following a fatal accident last month.
James Wright has Monday's forecast for the east of the region.
James Wright has Monday's forecast for the west of the region.