Bath & North East Somerset Council is advising residents that Wessex Water sewer works are due to restart in Ambury and Corn Street in Bath on January 4.
Congestion for motorists is expected so residents and visitors are asked to plan journeys in advance, avoiding the area where possible or using alternative forms of travel.
The works are to enable the development of Bath Quays North. The sewer currently runs under the car park but is being diverted into the highway within Ambury and Corn Street.
Works are scheduled to take six weeks and are expected to be completed by February 11.
The right-hand carriageway of the Ambury remains open, but the left turn into Corn Street will be closed, preventing vehicles from turning left into Corn Street.
Vehicles approaching the city from the south side of the river should follow the diversion along the Lower Bristol Road and via Midland Bridge Road.
Access to Avon Street car park will be via Green Park Road and not via the Ambury. Diversion signs will be in place.
The bus stops on the Ambury will remain operational and buses will follow the diversion.
Wessex Water has notified adjacent residents and businesses of the works.