The work involves repairing cracks, strengthening the bridge arch and abutments, and fixing sections of the canal wall below the water level.
The work will take just under four months to complete, with navigation along the canal set to resume on 11 March, and the road due to reopen in mid-April. Diversions for road and towpath users will be signposted.
New lock gates
The project forms part of a £45 million programme of winter maintenance works on the country's canals and rivers, which will also include replacing multi-tonne lock gates at historic Seend Lock in the new year.
Mark Evans, waterway manager at the Canal & River Trust said: “The repairs we're doing to Seend Wharf Bridge are absolutely essential. I know that closing the bridge over Christmas is going to be annoying for people, but safety comes first and we're at the stage where putting this off isn't an option.
"We really appreciate people's patience while we get this done, and we'll do everything we can to keep people in the loop and update them on how the work is going.”