Bargates will be shut off to through traffic in one direction from Tuesday 25 January, while repair work takes place on a short stretch of the road close to the Fountain Roundabout.
The substantial nature of the project, with the sewer more than five metres below ground, means the one-lane closure will be in place between Stour Road and Fountain Way until the expected finish date of Friday 22 April.
A diversion from the junction of Stour Road at the northern end of Bargates, will be in place via Stour Road and Barrack Road - with a banksman in place between 7am-7pm to offer help and guidance to road users and signs in place to explain that businesses on the road remain open as usual.
Access will be maintained for business customers and deliveries on the road as well as residents, although they will not be able to exit onto the Fountain roundabout from Bargates due to the work taking place.
However, those heading away from the town centre will still be able to access Bargates from Fountain Roundabout, while the on-street parking further up the road will be unaffected by the repairs.
Wessex Water initially carried out an emergency repair on the collapsed sewer last year, laying temporary pipework to ensure sewerage services were maintained for residents and businesses on the road.
A company spokesperson said: “We know there has been concern around the scope of this work on the collapsed sewer from both businesses and residents and we want to reassure them that while this is a significant repair, every effort has been made to minimise the impacts on them as much as possible.
“Local people and businesses have been informed about the work that needs to be carried out and they, their customers and deliveries will still have access throughout the repair. “Bargates can still be accessed from the Fountain roundabout heading away from the town centre in the south with a diversion in place from the junction of Stour Road in the north for the one-lane closure.
“We’re sorry for any inconvenience caused by the road closure but it is necessary to ensure this work can be carried out to the collapsed sewer safely and we’ll be working hard to complete this work as quickly as possible.’’