The bridge carrying the A47 over the River Great Ouse at King’s Lynn will be closed overnight for three weeks leaving drivers facing a 25-mile diversion.
The closure is to allow Highways England to carry out a routine inspection of the bridge structure.
Instead of using the bridge, motorists heading west will have to travel via Downham Market, using the A10 from Lynn’s Hardwick roundabout, along the A1122 and the A1101, before picking up the A47 at Wisbech.
Drivers travelling towards Lynn will follow the diversion in reverse, leaving the A47 at Wisbech.
The diversion will add around 10 miles to people’s journeys.
A Highways England spokesman said: “We are planning inspection work early next year. We will carry the work out overnight, with all restrictions removed during the day.
“We have discussed with the council whether it is possible to use a shorter diversion though King’s Lynn, but unfortunately it is not.”
Regular inspections help Highways England to monitor the condition of the bridge and plan maintenance to keep journeys across it safe and reliable for road-users.
The work is scheduled for February but is subject to an ecological survey being carried out next month to see if bats are living under the bridge.
If any bats are found then they will need to be re-homed before any work can be carried out.
“There will be one overnight closure in January for an ecological survey and then around three weeks of overnight closures, on week nights only, in February,” added the spokesman. “We will publish more details closer to the time and we encourage drivers to check ahead if they are planning to use the A47 overnight during this period.”
Norfolk county councillor Alexandra Kemp expressed concern about HGVs using Wisbech Road as a shortcut instead of following the diversion.
The road has a 7.5 tonne weight restriction in place and Miss Kemp called for overnight enforcement to be in place during the work.